CHAPTER V.

PORIFERA.

Although within the last few years greater advances have probably been made in our knowledge of the development of the Porifera than of any other group, yet there is much that is still very obscure, and it is not possible to make general statements applying to the whole group.

Calcispongiæ. The form which has so far been most completely worked out is _Sycandra raphanus_, one of the Calcispongiæ (Metschnikoff, Nos. 132 and 134, F. E. Schulze, Nos. 139 and 142), and I shall commence my account with the life history of this species.

The ovum in Sycandra as in other Spongida has the form of a naked amoeboid nucleated mass of protoplasm. From the analogy of the other members of the group, there is no doubt that it is fertilized by a male spermatic element, though this has not as yet been shewn to be the case--and the changes which accompany fertilization are quite unknown.

[FIG. 63. SUCCESSIVE STAGES IN THE SEGMENTATION OF SYCANDRA RAPHANUS. (Copied from F. E. Schulze.)

A. stage with eight segments still arranged in pairs, from above. B. side view of stage with eight segments. C. side view of stage with sixteen segments. D. side view of stage with forty-eight segments. E. view from above of stage with forty-eight segments. F. side view of embryo in the blastosphere stage, eight of the granular cells which give rise to the epiblast of the adult are present at the lower pole.

_cs._ segmentation cavity; _ec._ granular cells which form the epiblast; _en._ clear cells which form the hypoblast.]

The segmentation and early stages of development take place in the tissues of the parent. The segmentation is somewhat peculiar, though a modification of a regular segmentation. The ovum divides along a vertical plane, first into two, and then into four equal segments. But even when two segments are formed, each of them has one end pointed and the other broader. The pointed ends give rise to the ciliated cells of the future larva, and the broad ends to the granular cells. Instead of the next division taking place, as is usually the case, in a horizontal (equatorial) plane, it is actually effected along two vertical planes intermediate in position between the two first planes of segmentation. Eight equal segments are thus formed, each of which has the form of a pyramid. All the segments are situated in a single tier, and are so arranged as to give to the whole ovum the form of a flat cone, the apex of which is formed by the pointed extremities of the constituent segments (fig. 63 B). The apices of the segments do not however quite meet, but they leave a central space, which is an actual perforation (fig. 63 A) through the axis of the ovum, open at both ends. The first indications of this perforation appear when only four segments are present, and it is to be regarded as the homologue of the segmentation cavity of other ova. The next plane of division is horizontal (equatorial), and the apices of the eight cells are segmented off as a tier of small cells. At the completion of this division (fig. 63 C), the ovum is formed of sixteen cells arranged in two superimposed tiers. The ovum now assumes somewhat the form of a biconvex lens, in the axis of which the central perforation is still present. At the close of the next stage, forty-eight cells are present arranged in four tiers (fig. 63 D and E), the two outer tiers containing eight cells each, and the two inner sixteen. The two inner tiers probably arise by the simultaneous appearance of two equatorial furrows dividing the original tiers into two, and by the subsequent simple division of the cells of the two inner of the tiers so formed. At the close of the stage the eight basal cells become granular (fig. 63 F). At the same time the central part of the segmentation cavity becomes enlarged, while its terminal apertures become narrowed and finally, shortly after the end of this stage, closed. The axial perforation thus acquires the character of a closed segmentation cavity. While the ovum itself becomes at the same time a blastosphere.

[FIG. 64. LARVA OF SYCANDRA RAPHANUS AT PSEUDOGASTRULA STAGE, IN SITU IN THE MATERNAL TISSUES. (Copied from F. E. Schulze.)

_me._ mesoblast of adult; _hy._ collared cells forming hypoblast of the adult; _en._ clear cells of larva which eventually become involuted to form the hypoblast; _ec._ granular cells of larva which give rise to the epiblast, which at this stage are partially involuted.]

This stage nearly completes the segmentation: in the next one, the cells of the poles of the blastosphere increase in number, and the cells of the greater part of the blastosphere become columnar and ciliated, (fig. 64 _en._) while the granular cells (_ec._) increase to about thirty-two in number and appear to be (partially at least) involuted into the segmentation cavity, reducing this latter to a mere slit. This stage forms the last passed by the embryo in the tissues of the parent. The general position of the embryo while still in this situation may be gathered from fig. 64, representing the embryo _in situ_. The embryo is always placed close to one of the radial canals. From this situation it makes its way through the lining cells into a canal and is thence transported to the surrounding water. By the time the larva has become free, the semi-invaginated granular cells have increased in bulk and become everted so as to project very much more prominently than in the encapsuled state. To the gastrula stage, if it deserves the name, passed through by the embryo in the tissues of the parent, no importance can be attached.

[FIG. 65. TWO FREE STAGES IN THE DEVELOPMENT OF SYCANDRA RAPHANUS. (Copied from Schulze.)

A. Amphiblastula stage. B. A later stage after the ciliated cells have commenced to become invaginated. _cs._ segmentation cavity; _ec._ granular cells which will form the epiblast; _en._ ciliated cells which become invaginated to form the hypoblast.]

The larva, after it has left the parental tissues, has an oval form and is transversely divided into two areas (fig. 65 A). One of these areas is formed of the elongated, clear, ciliated cells, with a small amount of pigment near their inner ends (_en._), and the other and larger area of the thirty-two granular cells already mentioned (_ec._). Fifteen or sixteen of these are arranged as a special ring on the border of the clear cells. In the centre of the embryo is a segmentation cavity (_c.s._) which lies between the granular and the clear cells, but is mainly bounded by the vaulted inner surface of the latter. This stage is known as the amphiblastula stage. During the later periods of the amphiblastula stage a cavity appears in the granular cells dividing them into two layers. After the larva has for some time enjoyed a free existence, a remarkable series of changes take place, which result in the invagination of the half of it formed of the clear cells, and form a prelude to the permanent attachment of the larva. The entire process of invagination is completed in about half an hour. The whole embryo first becomes flattened, but especially the ciliated half, which gradually becomes less prominent (fig. 65 B); and still later the cells composing it undergo a true process of invagination. As a result of this invagination the segmentation cavity is obliterated, and the larva assumes a compressed plano-convex form, with a central gastrula cavity, and a blastopore in the middle of the flattened surface. The two layers of the gastrula may now be spoken of as epiblast and hypoblast. The blastopore becomes gradually narrowed by the growth over it of the outer row of granular cells. When it has become very small the attachment of the larva takes place by the flat surface where the blastopore is situated. It is effected by protoplasmic processes of the outer ring of epiblast cells, which, together with the other epiblast cells, now become amoeboid. They become at the same time clearer and permit a view of the interior of the gastrula. Between the epiblast cells and the hypoblast cells which line the gastrula cavity there arises a hyaline structureless layer, which is more closely attached to the epiblast than to the hypoblast, and is probably derived from the former. A view of the gastrula stage after the larva has become fixed is given in fig. 66.

[FIG. 66. FIXED GASTRULA STAGE OF SYCANDRA RAPHANUS. (Copied from Schulze.)

The figure shews the amoeboid epiblast cells (_ec._) derived from the granular cells of the earlier stage, and the columnar hypoblast cells, lining the gastrula cavity, derived from the ciliated cells of the earlier stage. The larva is fixed by the amoeboid cells on the side on which the blastopore is situated.]

There would seem according to Metschnikoff's observations (No. 134) to be a number of mesoblast cells interposed between the two primary layers, which he derives from the inner part of the mass of granular cells.

After invagination the cilia of the hypoblast cells can no longer be seen, and are probably absorbed; and their disappearance is nearly coincident with the complete obliteration of the blastopore, an event which takes place shortly after the attachment of the larva.

Not long after the closure of the blastopore, calcareous spicules make their appearance in the larva as delicate unbranched rods pointed at both extremities. They appear to be formed on the mesoblast cells situated between the epiblast and hypoblast[67]. The larva when once fixed rapidly grows in length and assumes a cylindrical form (fig. 67 A). The sides of the cylinder are beset with calcareous spicules which project beyond the surface, and, in addition to the unbranched forms, spicules are developed with three and four rays as well as some with a blunt extremity and serrated edge. The extremity of the cylinder opposite the attached surface is flattened, and, though surrounded by a ring of four-rayed spicules, is itself free from them. At this extremity a small perforation is formed leading into the gastric cavity, which rapidly increases in size and forms an exhalent osculum (_os._). A series of inhalent apertures is also formed at the sides of the cylinder. The relative times of appearance of the single osculum and the smaller apertures are not constant for the different larvæ. On the central gastrula cavity of the sponge becoming placed in communication with the external water, the hypoblast cells lining it become ciliated afresh (fig. 67 B, _en._) and develop the peculiar collar characteristic of the hypoblast cells of the Spongida (_vide_ fig. 64, _hy._). When this stage of development is reached we have a fully formed sponge of the type made known by Haeckel as Olynthus.

[67] Metschnikoff was the first to give this account of the development of the spicules in Sycandra, but Prof. Schulze has informed me by letter that he has arrived at the same result.

[FIG. 67. THE YOUNG OF SYCANDRA RAPHANUS SHORTLY AFTER THE DEVELOPMENT OF THE SPICULA. (Copied from Schulze.)

A. View from the side. B. View from the free extremity. _os._ osculum; _ec._ epiblast; _en._ hypoblast composed of ciliated cells. The terminal osculum and lateral pores are represented as oval white spaces.]

When young examples of Sycandra come in contact shortly after their attachment they appear to fuse together temporarily or else permanently. In the latter case colonies are produced by their fusion.

Amongst other calcareous sponges the larva of _Ascandra contorta_ (Haeckel No. 126, Barrois No. 122) presents the typical amphiblastula stage, and so probably does that of _Ascandra Lieberkühnii_ (Keller No. 128). In _Leucandra aspera_ (Keller No. 128, Metschnikoff No. 134) the larva passes through an amphiblastula stage, but the characters of the cells of the two halves of the larva do not differ to nearly the same extent as in Sycandra.

Although the majority of calcareous sponges appear to agree in their mode of development with Sycandra, nevertheless the concordant researches of O. Schmidt (No. 138) and Metschnikoff (No. 134) have shewn that this is not true for the genus Ascetta (_As. primordialis_, _clathrus_ and _blanca_).

The larvæ of these forms are very differently constituted to those of Sycandra. They have an oval form and are composed of a single row of ciliated columnar cells: their two extremities only differ in the cells at one extremity being longer than those at the other. Especially at the pole where the shorter cells are situated (Schmidt) a metamorphosis of the cells takes place. One after the other they lose their cilia, become granular, and pass into the interior of the vesicle. Here they become differentiated into two classes (Metschnikoff); one of larger and more granular cells, and the other of smaller cells with clearer protoplasm. Cells of the former class are mainly found at one of the poles. When the larva becomes free the cells in the interior of the vesicle increase in number and nearly fill up its central cavity. After a short free existence the larva becomes fixed, and the epiblast cells lose their cilia and become flattened. At a later period the large granular cells assume a radiate arrangement round a central cavity and become clearly marked out as the hypoblast cells. The smaller cells become placed between the epiblast and hypoblast and constitute the mesoblast.

Myxospongiæ. In this group Halisarca has been investigated by Carter (No. 123), Barrois (No. 122), Schulze (No. 141) and Metschnikoff (No. 134). The ova develop in the mesoblast, and when ripe occupy special chambers lined by a layer of epithelial cells. Schulze has found the spermatozoa of this genus of sponge and has been able to shew that the sexes may be distinct, though many species of Halisarca are hermaphrodite.

The segmentation is, roughly speaking, regular, and a segmentation cavity is early formed, which is never, as in Calcispongiæ, open at the poles. When the larva leaves the parent it is an oval vesicle formed of a single layer of columnar ciliated cells. Slight differences may be observed between the two extremities of the larvæ of most species. One of these--the hinder extremity--is directed backwards in swimming.

The further history of the larva has been investigated by Metschnikoff. He has found that the interior of the vesicle becomes gradually filled with mesoblast cells of a peculiar type, called by him rosette-cells, which are probably derived from the walls of the vesicle.

When the metamorphosis commences, the larva assumes a flattened form, and cells of a new type, viz. normal amoeboid cells, grow in amongst the rosette cells. The new cells are also derived from the epiblast. The larvæ appear to fix themselves by the hinder extremity. The cilia gradually disappear, and the epiblast cells flatten out and form a kind of cuticle. For some time the larva remains in the two-layered condition, but gradually canals (? ciliated chambers) lined by hypoblast cells become formed. They appear as closed spaces with walls of ciliated cells derived from the amoeboid cells, and the different parts of the system of chambers are established independently. In _H. pontica_ the ciliated chambers are formed before the attachment of the larva. The development was not followed up to the formation of the pores placing the canal system in communication with the exterior.

The young sponges at a somewhat later stage have been studied by Schulze and Barrois. They are formed of an external layer of flattened cells, not clearly ciliated as in the adult, within which are a normal mesoblastic tissue, and several spherical chambers lined by ciliated cells exactly like the ciliated chambers of the full-grown sponge. Irregular invaginations of the epiblast give to the young sponge a honeycombed structure. The ciliated chambers in the youngest condition of the sponge are closed; but in slightly older examples they come into communication with the passages lined by hypoblast, and so indirectly with the external medium.

Ceratospongiæ. Amongst the true Ceratospongiæ the embryos of two of the Aplysinidæ, and of Spongelia and Euspongia have been to some extent worked out by Barrois and Schulze. The form worked out by Barrois is called by him _Verongia rosea_. The segmentation is nearly regular, but from the first the segments may be divided according to their constitution into two categories. At the close of segmentation the embryo is oval and covered by a single layer of columnar ciliated cells; these cells may however be divided into two categories, corresponding with those observable during the segmentation. A certain number are coloured red and form a definite circular mass at one pole, while the remainder, which constitute the major part of the embryo, have a pale yellowish colour. Those at the red pole lose their cilia in the free larva, but around the area formed by them is a special ring of long cilia. The chief peculiarity of the embryo (made known by Schulze) consists in the fact that the layer of cells which covers the embryo does not, as in other sponge embryos, simply enclose a space, but the interior of the embryo is formed of a mass of stellate cells like the normal mesoblast of full-grown sponges.

This feature is also characteristic of the embryos of Spongelia and Euspongia.

The embryo of the Gummineæ (_Gummina mimosa_) has been investigated by Barrois (No. 122), and has been shewn closely to resemble the typical larvæ of calcareous sponges; one-half being formed of _elongated ciliated cells_ and the other of rounded granular ones.

Silicispongiæ. The development of marine silicious sponges is but very imperfectly understood. The larvæ of various forms--Reniera (Isodyctia), Esperia (Desmacidon), Raspailia, Halichondria, Tethya--have been described. Barrois has shewn that the egg segments regularly and that in the earlier stages a segmentation cavity is present. In the later stages the embryo appears to become solid. Externally there is a layer of ciliated cells, and within a mass of granular matter in which the separate cells cannot be made out. The granular matter projects at one pole, and forms a prominence possibly equivalent to the granular cells of Sycandra. In some forms, _e.g._ Reniera, the edge of the unciliated granular prominence may be surrounded by a row of long cilia. In later stages the granular material may project at both poles or even at other points. One remarkable feature in the development of the Silicispongiæ is the appearance of spicula between the ciliated cells and the central mass, while the larva is still free.

Professor Schulze has informed me that these spicula are developed in mesoblast cells; while the horny fibres of the sponge are developed as cuticular products of special mesoblast cells (spongioblasts).

The attachment and accompanying metamorphosis are so diversely described that no satisfactory account can be given of them. The general statements are in favour of the attachment taking place by the posterior extremity where the granular matter projects.

Carter especially gives a very precise account, with figures, of the attachment of the larva in this way. He also figures the appearance of an osculum at the opposite pole[68].

[68] Keller (No. 129) has recently given an account of the development of Halichondria (Chalinula) fertilis. He finds that there is an irregular segmentation, followed by a partial epibolic invagination, the inner mass of cells remaining exposed at one pole and forming there a prominence, equivalent to the granular prominence in the larvæ of other Silicispongiæ. The free-swimming larva resembles the larva of other Silicispongiæ in the possession of spicula, etc., and after becoming laterally compressed attaches itself by one of the flattened sides. A central cavity is formed in the interior with ciliated chambers opening into it, and is subsequently placed in communication with the exterior by the formation of an aperture which constitutes the osculum.

A very elaborate account of the development of Spongilla has been published in Russian by Ganin, of which a German abstract has also appeared (No. 124).

The ovum undergoes a regular segmentation and becomes a solid ova morula. An epiblast of smaller cells is early differentiated, and in the interior of the inner cells an archenteron becomes subsequently formed. The inner cells next become divided into an hypoblastic layer lining the archenteron, and a mesoblastic layer between this and the now ciliated epiblast. At the narrow hinder end of the embryo the mesoblast becomes thickened, and largely obliterates the archenteron. In this part of the mesoblast silicious spicula are formed. The larva becomes attached by its hinder extremity, and in the course of this process flattens itself out to a disc-like form. From the nearly obliterated archenteric cavity outgrowths take place which give rise to the ciliated chambers. These are not placed directly in communication with the exterior, but open, if I understand Ganin rightly, into a space in the mesoblast, which subsequently acquires an exterior communication--the primitive osculum. The subsequent pores and oscula are also formed as openings leading into the mesoblastic cavity, which communicates in its turn with the ciliated chambers.

It appears that in the present unsatisfactory state of our knowledge the larvæ of the Porifera may be divided into two groups: viz. (1) those which have the form of a blastosphere or else of a solid morula; (2) those which have the amphiblastula form.

In the former type the mesoblast and hypoblast are formed either from cells budded off from the outer cells of the blastosphere or from the solid inner mass of cells; while the outer ciliated cells become the epiblast. This type of larva, which is found in the majority of sponges, is very similar in its general characters and development to many Coelenterate planulæ.

The second type of larva is very peculiar, and though in its fully developed form it is confined to the Calcispongiæ, where it is the usual form, a larval type with the same characters is perhaps to be found in other sponges, _e.g._ amongst the Gumminæ, and amongst the Silicispongiæ where one-half of the embryo is without cilia, though in the case of the Silicispongiæ the cells of the ciliated part of the embryo correspond to the granular cells of the larva of Sycandra.

The later stages in the development of the larvæ of the Porifera are not similar to anything we know of in other groups.

It might perhaps be possible to regard sponges as degraded descendants of some Actinozoon type such as Alcyonium, with branched prolongations of the gastric cavity, but there does not appear to me to be sufficient evidence for doing so at present. I should rather prefer to regard them as an independent stock of the Metazoa.

In this connection the amphiblastula larva presents some points of interest. Does this larva retain the characters of an ancestral type of the Spongida, and if so, what does its form mean? It is, of course, possible that it has no ancestral meaning but has been secondarily acquired; but, assuming that this is not the case, it appears to me that the characters of the larva may be plausibly explained by regarding it as a transitional form between the Protozoa and Metazoa. According to this view the larva is to be considered as a colony of Protozoa, one-half of the individuals of which have become differentiated into nutritive forms, and the other half into locomotor and respiratory forms. The granular amoeboid cells represent the nutritive forms, and the ciliated cells represent the locomotor and respiratory forms. That the passage from the Protozoa to the Metazoa may have been effected by such a differentiation is not improbable on _a priori_ grounds.

While the above view seems fairly satisfactory for the free-swimming stage of the larval sponge, there arises in the subsequent development a difficulty which appears at first sight fatal to it. This difficulty is the invagination of the ciliated cells instead of the granular ones. If the granular cells represent the nutritive individuals of the colony, they, and not the ciliated cells, ought most certainly to give rise to the lining of the gastrula cavity, according to the generally accepted views of the morphology of the Spongida. The suggestion which I would venture to put forward in explanation of this paradox involves a completely new view of the nature and functions of the germinal layers of adult Spongida.

It is as follows:--When the free-swimming ancestor of the Spongida became fixed, the ciliated cells by which its movements used to be effected must have to a great extent become functionless. At the same time the amoeboid nutritive cells would need to expose as large a surface as possible. In these two considerations there may, perhaps, be found a sufficient explanation of the invagination of the ciliated cells, and the growth of the amoeboid cells over them. Though respiration was, no doubt, mainly effected by the ciliated cells, it is improbable that it was completely localized in them, but they were enabled to continue performing this function through the formation of an osculum and pores. The collared cells which line the ciliated chambers, or in some cases the radial tubes, are undoubtedly derived from the invaginated cells, and, if there is any truth in the above suggestion, the collared cells in the adult sponge must be mainly respiratory and not digestive in function, while the epiblastic cells, which in most cases line the inhalent passages through its substance[69], ought to be employed to absorb nutriment. The recent researches of Metschnikoff (No. 134) on this head shew that the nutriment is largely carried into the mesoblast cells, which in Sycandra appear to be derived from the granular cells, and also that it is taken up by the cells which line the passages, though not by the superficial epiblast cells. Whether the collared cells generally absorb nutriment is not clear from his statements: but _he finds that they do not do so in Silicispongiæ_.

[69] That the greater part of the flat cells which line the passages of most Sponges are really derived from epiblastic invaginations appears to me to be proved by Schulze's and Barrois' observations on the young fixed stages of Halisarca. Schulze's (No. 140) observations have however proved that the flat cells lining the axial gastric chamber of Sycandra are hypoblastic in origin, and the observations of Keller (No. 129) and Ganin (No. 124) have led to the same result for the flat epithelium lining part of the passages of the Silicispongiæ.

Professor Schulze has informed me by letter that he finds the collared cells to be respiratory in function, while the cells derived from the granular cells in Sycandra are nutritive. Carter[70], on the contrary, from his observations on Spongilla, has fully satisfied himself that the food is absorbed by the cells lining the ciliated chambers.

[70] "On the Nutritive and Reproductive Processes of Sponges." _Ann. and Mag. of Nat. Hist._, Vol. IV. Ser. V. 1879.

If it is eventually proved by further experiments on the nutrition of sponges, that digestion is mainly carried on by the general cells lining the passages and the mesoblast cells, and not for the most part by the ciliated cells, it is clear that the epiblast, mesoblast and hypoblast of sponges will not correspond with the similarly named layers in the Coelenterata and other Metazoa. The invaginated hypoblast will be the respiratory layer and the epiblast and mesoblast the digestive and sensory layers; the sensory function being probably mainly localized in the epithelium on the surface, and the digestive one in the epithelium lining the passages and in the mesoblast. Such a fundamental difference in the primary function of the germinal layers between the Spongida and the other Metazoa, would necessarily involve the creation of a special division of the Metazoa for the reception of the former group.

BIBLIOGRAPHY.

(122) C. Barrois. "Embryologie de quelques éponges de la Manche." _Annales des Sc. Nat. Zool._, VI. ser., Vol. III. 1876.

(123) Carter. "Development of the Marine Sponges." _Annals and Mag. of Nat. Hist._, 4th series, Vol. XIV. 1874.

(124) Ganin[71]. "Zur Entwicklung d. Spongilla fluviatilis." _Zoologischer Anzeiger._ Vol. I. No. 9, 1878.

(125) Robert Grant. "Observations and Experiments on the Structure and Functions of the Sponge." _Edinburgh Phil. J._, Vol. XIII. and XIV., 1825, 1826.

(126) E. Haeckel. _Die Kalkschwämme_, 1872.

(127) E. Haeckel. _Studien zur Gastræa-Theorie._ Jena, 1877.

(128) C. Keller. _Untersuchungen über Anatomie und Entwicklungsgeschichte einiger Spongien._ Basel, 1876.

(129) C. Keller. "Studien üb. Organisation u. Entwick. d. Chalineen." _Zeit. f. wiss. Zool._, Bd. XXVIII. 1879.

(130) Lieberkühn. "Beitr. z. Entwick. d. Spongillen." Müller's _Archiv_, 1856.

(131) Lieberkühn. "Neue Beiträge zur Anatomie der Spongien." Müller's _Archiv_, 1859. (132) El. Metschnikoff. "Zur Entwicklungsgeschichte der Kalkschwämme." _Zeit. f. wiss. Zool._, Bd. XXIV. 1874.

(133) El. Metschnikoff. "Beiträge zur Morphologie der Spongien." _Zeit. f. wiss. Zool._, Bd. XXVII. 1876.

(134) El. Metschnikoff. "Spongeologische Studien." _Zeit. f. wiss. Zool._, Bd. XXXII. 1879.

(135) Miklucho-Maklay. "Beiträge zur Kenntniss der Spongien." _Jenaische Zeitschrift_, Bd. IV. 1868.

(136) O. Schmidt. "Zur Orientirung über die Entwicklung der Schwämme." _Zeit. f. wiss. Zool._, Bd. XXV. 1875.

(137) O. Schmidt. "Nochmals die Gastrula der Kalkschwämme." _Archiv für mikrosk. Anat._, Bd. XII. 1876.

(138) O. Schmidt. "Das Larvenstadium von Ascetta primordialis und Asc. clathrus." _Archiv für mikrosk. Anatomie_, Bd. XIV. 1877.

(139) F. E. Schulze. "Ueber den Bau und die Entwicklung von Sycandra raphanus." _Zeit. f. wiss. Zool._, Bd. XXV. 1875.

(140) F. E. Schulze. "Zur Entwicklungsgeschichte von Sycandra." _Zeit. f. wiss. Zool._, Bd. XXVII. 1876.

(141) F. E. Schulze. "Untersuchung üb. d. Bau, etc. Die Gattung Halisarca." _Zeit. f. wiss. Zool._, Bd. XXVIII. 1877.

(142) F. E. Schulze. "Untersuchungen üb. d. Bau, etc. Die Metamorphose von Sycandra raphanus." _Zeit. f. wiss. Zool._, Bd. XXXI. 1878.

(143) F. E. Schulze. "Untersuchungen ü. d. Bau, etc. Die Familie Aplysinidæ." _Zeit. f. wiss. Zool._, Bd. XXX. 1878.

(144) F. E. Schulze. "Untersuchungen ü. d. Bau, etc. Die Gattung Spongelia." _Zeit. f. wiss. Zool._, Bd. XXXII. 1878.

[71] There is a Russian paper by the same author, containing a full account, with clear illustrations, of his observations.