CHAPTER IX.

DETAILS OF COELENTERATA.

I. HYDROZOA.

_A. Hydrida._

The Hydras, as a rule, are not coloured in our sense of the term; that is to say, they are of a general uniform brown colour. But in one species, _H. viridis_, the endoderm contains granules of a green colour, which is said to be identical with the green colouring matter of leaves (_chlorophyll_). This does not occur in all the cells, though it is present in most. The green matter occurs in the form of definite spherical corpuscles, and these colour-cells define the inner layer of the integument (the endoderm), and render it distinct.[22] That portion of the endoderm which forms the boundary of the body-cavity has fewer green corpuscles, but contains irregular brown granules, thus roughly mapping out a structural region.

We thus see that even in so simple a body as the Hydra the colouring matter is distributed strictly according to morphological tracts.

_B. Tubularida._ The Tubularian Hydroids are the subject of an exhaustive and admirably illustrated monograph by Prof. J. Allman, from which the following details are culled. These animals are with few exceptions marine, and consist either of a single polypite or of a number connected together by a common flesh, or coenosarc. Some are quite naked, others have horny tubes, into which, however, the polypites cannot retreat. The polypites consist essentially of a sac surrounded with tentacles; and one of their most striking characters is their mode of reproduction. Little buds (_gonophores_) grow from the coenosarc, and gradually assume a form exactly like that of a jelly-fish. These drop off, and swim freely about; and are so like jelly-fishes that they have been classed among them as separate organisms.

The Tubulariæ are all transparent; and in them we find structural colouration finely shown, the colour, as is usual in transparent animals, being applied directly to the different organs.

Writing of the colour, Prof. Allman says: "That distinct secretions are found among the Hydroida, and that even special structures are set aside for their elaboration, there cannot now be any doubt.

"One of the most marked of these secretions consists of a coloured granular matter; which is contained at first in the interior of certain spherical cells, and may afterwards become discharged into the somatic fluid. These cells, as already mentioned, are developed in the endoderm;[23] in which they are frequently so abundant as to form a continuous layer upon the free surface of this membrane. It is in the proper gastric cavity of the hydranth and medusa, in the spadix of the sporosac, and in the bulbous dilatations which generally occur at the bases of the marginal tentacles of the medusæ, that they are developed in greatest abundance and perfection; but they are also found, more or less abundantly, in the walls of probably the whole somatic cavity, if we except that portion of the gastrovascular canals of the medusa which is not included within the bulbous dilatations.

"In the parts just mentioned as affording the most abundant supply of these cells, they are chiefly borne on the prominent ridges into which the endoderm is thrown in these situations; when they occur in the intervals between the ridges they are smaller, and less numerous.

"The granular matter contained in the interior of these cells varies in its colour in different hydroids. In many it presents various shades of brown; in others it is a reddish-brown, or light pink, or deeper carmine, or vermilion, or orange, or, occasionally, a fine lemon-yellow, as in the hydranth of _Coppinia arcta_, or even a bright emerald green, as in the bulbous bases of the marginal tentacles of certain medusæ. No definite structure can be detected in it; it is entirely composed of irregular granules, irregular in form, and usually aggregated into irregularly shaped masses in the interior of the cells. It is to this matter that the colours of the _Hydroida_, varying, as they do, in different species, are almost entirely due.

"The coloured granular matter is undoubtedly a product of true secretion; and the cells in which it is found must be regarded as true secreting cells. These cells are themselves frequently to be seen as secondary cells in the interior of parent cells, from which they escape by rupture, and then, falling into the somatic fluid, are carried along by its currents, until, ultimately, by their own rupture, they discharge into it their contents.

"We have no facts which enable us to form a decided opinion as to the purpose served by this secretion. Its being always more or less deeply coloured, and the fact of its being abundantly produced in the digestive cavity, might suggest that it represented the biliary secretion of higher animals. This may be its true nature, but as yet we can assert nothing approaching to certainty on the subject; indeed, considering how widely the cells destined for the secretion of coloured granules are distributed over the walls of the somatic cavity, it would seem not improbable that the import of the coloured matter may be different in different situations; that while some of it may be a product destined for some further use in the hydroid, more of it may be simply excretive, taking no further part in the vital phenomena, and intended solely for elimination from the system."[24]

Here we have very definite statements by a highly trained observer of the distribution of colour in the whole of these animals, and of the conclusions he draws from them.

Firstly as to the colour itself. We find it true colour--brown, pink, carmine, vermilion, orange, lemon-yellow, and even emerald green; a set of hues as vivid as any to be found in the animal kingdom. It is difficult to conceive these granules to be merely excrementitious matter; for in such simple creatures, feeding upon such similar bodies, one would hardly expect the excretive matter to be so diversified in tint. Moreover, excrementitious matter is not, as a rule, highly coloured, but brown. Thus, we see in the Rhizopods the green vegetable matter which has been taken in as food becomes brown as the process of assimilation goes on; and, indeed, colour seems almost always to be destroyed by the act of digestion.

Still, it by no means follows that this colour, even if it is produced for the sake of decoration, as we suggest, may not owe its direct origin to the process of digestion. The digestive apparatus is the earliest developed in the animal kingdom, and in these creatures is by far the most important; the coelenterata being, in fact, little more than living stomachs. If, then, colouration be structural, what is more likely than that the digestive organs should be the seat of decoration in such transparent creatures?

Secondly, as to the distribution of the colour. We find it "frequently forming a continuous layer upon the free surface of" the endoderm, in the "spadix of the sporosac," and in the "bulbous terminations" of the canals, that colour is best developed. In other words, the colour is distributed structurally, and is most strongly marked where the function is most important.

Prof. Allman gives no hint that the colour may be purely decorative, and is naturally perplexed at the display of hues in such vigour; but if this be one of the results of the differentiation of parts, of the specialization of function, then we can, at least, understand why we find such brilliant colour in these creatures, and why it is so distributed.

As an illustration of the _Tubularia_ we have selected _Syncoryne pulchella_, Fig. 2, Pl. VI., and its medusa, Fig. 1. The endoderm of the spadix of the hydranths is of a rich orange colour, which becomes paler as it descends towards the less highly organized stem. Medusæ are seen in various stages of development, and one, mature and free, is shown. In these the manubrium, and the bulbous terminations of the canals are also seen to be coloured orange.

In these medusæ we find the first appearance of sensory organs. They consist of pigment-cells enclosed in the ectoderm, or outside covering; and are singular as presenting the first true examples of opaque colouring in the animal kingdom. They are associated with nerve cells attached to a ring of filamentous nerve matter, surrounding the base of the bell. In some important respects the pigment differs from that in other parts of the animal. It is more definite in structure; and the whole ocellus is "aggregation of very minute cells, each filled with a homogeneous coloured matter."[25] These ocelli, and similar sense organs, called _lithocysts_, are always situated over the bulbous termination of the canals. The pigment is black (as in this case), vermilion, or deep carmine.

The dependence of colour upon structure is thus shown to hold good throughout these animals in a most remarkable manner, and the acceptance of the views here set forth gives us an insight into the reasons for this colouration which, as we have seen, did not arise from the study of the question from the ordinary point of view.

_C. Sertularida._ These animals are very similar to the last, but they are all compound, and the polypites can be entirely withdrawn within the leathery investment or polypary. Their mode of reproduction is also similar, and their colouration follows the same general plan. Being so like the preceding order, it is unnecessary to describe them.

_B. Siphonophora._

The Siphonophora are all free-swimming, and are frequently called Oceanic Hydrozoa. They are divided into three orders, viz.:--

_a. Calycophoridæ._ _b. Physophoridæ._ _c. Medusidæ._

_a. Calycophoridæ._ These animals have a thread-like coenosarc, or common protoplasm, which is unbranched, cylindrical, and contractile. They are mostly quite transparent, but where colour exists it is always placed structurally. Thus, in _Diphyes_ the sacculi of the tentacles are reddish, in _Sphæronectes_ they are deep red, and in _Abyla_ the edges of the larger specimens are deep blue.[26]

_b. Physophoridæ._ These creatures are distinguished by the presence of a peculiar organ, the float, or _pneumatophore_, which is a sac enclosing a smaller sac. The float is formed by a reflexion of both the ectoderm and endoderm, and serves to buoy up the animal at the surface of the sea. The best known species is the Physalia, or Portuguese Man-o'-War.

Prof. Huxley, in his monograph on the Oceanic Hydrozoa, gives many details of the colouration; and, not having had much opportunity of studying them, the following observations are taken from his work. It will be seen that the Physophoridæ illustrate the structural distribution of colour in a remarkable manner.

_Stephanomia amphitridis_, the hydrophyllia, colourless, and so transparent as to be almost imperceptible in water, coenosarc whitish, enlarged portions of polypites, pink or scarlet, sacs of tentacles scarlet.

The enlarged portion of the polypites is marked with red striæ, "which are simply elevations of the endoderm, containing thread-cells and coloured granules." The small polypites do not possess these elevations, and are colourless.

_Agalma breve_, like a prismatic mass of crystal, with pink float and polypites.

_Athorybia rosacea_, float pink, with radiating dark-brown striæ, made up of dots; polypites lightish red, shading to pink at their apices; tentacles yellowish or colourless, with dark-brown sacculi; thread-cells dark brown.

_Rhizophysa filiformis_, pink, with deep red patch surrounding the aperture of the pneumatocyst.

_Physalia caravilla_, bright purplish-red, with dark extremities, and blue lines in the folds of the crest; polypites violet, with whitish points, larger tentacles red, with dark purple acetabula, smaller tentacles blue, bundles of buds reddish.

_P. pelagica_, in young individuals pale blue, in adult both ends green, with highest part of crest purple, tentacles blue, with dark acetabula; polypites dark blue, with yellow points.

_P. utriculus._ Prof. Huxley describes a specimen doubtfully referred to this species very fully, as follows:--

"The general colour of the hydrosoma is a pale, delicate green, passing gradually into a dark, indigo blue, on the under surface.

"The ridge of the crest is tipped with lake, and the pointed end is stained deep bluish-green about the aperture of the pneumatocyst.

"The bases of the tentacles are deep blue; the polypites deep blue at their bases, and frequently bright yellow at their apices; the velvetty masses of reproductive organs and buds on the under surface are light green."

He further remarks that the tentacles have reniform thickenings at regular intervals, and "the substance of each thickening has a dark blue colour, and imbedded within it are myriads of close-set, colourless, spherical thread-cells."

It would not be possible to find a more perfect example of regional colouration. Not only is each organ differently coloured, but the important parts of each organ, like the ridge of the crest, the bases of the tentacles, and the thread-cell bearing ridges of the tentacles, are also emphasized with deep colour.

_Velella._ This beautiful creature, which sometimes finds its way to our shores, is like a crystal raft fringed with tentacles, and having an upright oblique crest, or sail. The margins of the disk and crest are often of a beautiful blue colour, and the canals of the disk become deep blue as they approach the crest. The polypites may be blue, purple, green, or brown.

_C. Medusidæ._ The structure and colouration of the true Medusæ are so like that of the medusiform larvæ of the other Hydrozoa, that they need not be particularly described.

_D. Lucernarida._ Of this sub-class we need only cite the _Lucernaria_ themselves; which are pretty bell-shaped animals, having the power of attaching themselves to seaweeds, etc., and also of swimming freely about. Round the margin are eight tufts of tentacles, opposite eight lobes, the membrane between the lobes being festooned. In _L. auricula_, a British species, the membrane is colourless and transparent, the lobes bright red, or green, and the tentacles blue.

As a group the Hydrozoa display regional colouration in a very perfect manner.

II. ACTINOZOA.

It is not necessary to trace the colouration through all the members of this group, but we will trace the variation of colour through two species of anemonies, which have been admirably studied by Dr. A. Andres.[27] The first column shows the general hue, the second the tints of that hue which are sufficiently marked to form varieties as cochineal red, chocolate, bright red, rufous, liver-coloured, brown, olive, green and glaucous. The third column gives the spotted varieties, from which it will be seen that the chocolate, liver, and green coloured forms have each coloured varieties. It will be seen that the range of colour is very great, passing from pale pink, through yellowish-brown to blue-green.

-----------+-----------+-----------+---------------- Prevailing | Uniform | Spotted | colour. |varieties. |varieties. | Allied species. -----------+-----------+-----------+---------------- White. | ? | | A. candida. " | coccinea. | | " | chiocca. | tigrina. | Red. | rubra. | | " | rufosa. | | Yellow. | hepatica. | fragacea. | " | umbra. | | " | olivacea. | | " | viridis. | opora. | " | glaucus. | | Blue. | ? | | -----------+-----------+-----------+----------------

Varieties of Actinea Cari.

The following brief descriptions illustrate the distribution of the colour:--

_Actinea Cari._

Uniform varieties (_Homochroma_).

----------------------+---------------+----------------+--------+----------- | Column. | Tentacles. |Gonidia.| Zone. ----------------------+---------------+----------------+--------+----------- [alpha]. _Hepatica_ | red brown. | azure. | azure. | azure. [beta]. _Rubra_ | crimson. | violet. | |{wanting, [gamma]. _Chiocca_ | scarlet. | white. | |{or flesh | | | |{coloured. | | | | [delta]. _Coccinea_ | cochineal. | yellowish. | | [epsilon]. _Olivacca_ | olive-brown | azure. | azure. | | green. | | | [zeta]. _Viridis_ | green. | azure. | azure. | azure. | | | | Spotted varieties (_Heterochroma_). | | | | [eta]. _Tigrina_ |red, spotted | | | | yellow. | | | [theta]. _Fragacea_ |liver, spotted | | | | clear green. | azure or white.| |indistinct. [iota]. _Opora_ |green spotted, | | | | and striped | | | | yellow. | azure. | | ----------------------+---------------+----------------+--------+-----------

In this table the varieties above mentioned are further particularized. The column is the stalk or body, the tentacles are the arms, the gonidia the eye spots, and the zone the line round the base. It will be noticed that these regions are often finely contrasted in colour.

_Bunodes gemmaceus_ is another variable form, and the following varieties are recognised.

_Heterochroma._

[alpha]. Ocracea, } peristome ochre yellow, zone black, tentacles grey, (type) } with blue and white spots.

[beta]. _Pallida_, peristome whitish grey unbanded, tentacles with white spots.

[gamma]. _Viridescens_, peristome greenish white unbanded, tentacles with white spots and rosy shades.

[delta]. _Aurata_, column at base golden, peristome intenser yellow with crimson flush, tentacles grey with ochreous and white spots.

[epsilon]. _Carnea_, column at base flesh coloured, peristome rosy, tentacles rosy, with white spots.

_Homochroma._

[zeta]. _Rosea_, like [epsilon], but with rosy tubercles.

[eta] . _Nigricans_, peristome blackish, with blue and green reflexions (riflessi).

A few other examples may be given, all of which can be studied in Dr. André's magnificently coloured plates.

_Aiptasia mutabilis_ is yellow brown, the tentacles spotted in longitudinal rows, the spots growing smaller towards the tip, thus affording a perfect example of the adaptation of colour to structure.

_Anemonia sulcata_ has normally long light yellow pendulous tentacles tipped with rose, but a variety has the column still yellow but the tentacles pale green, tipped with rose.

_Bunodes rigidus_ has the column green, with rows of crimson tubercles, the tentacles are flesh-coloured, except the outer row which are pearly; the peristome is green, with brown lips.

[22] Allman's Hydroids. Ray. Soc., p. 123. [23] Compare with Hydra above. [24] Allman. Monograph of Tubularian Hydroida. Ray. Soc., p. 135. [25] Allman, _op. cit._, p. 139. [26] Huxley. Oceanic Hydrozoa, pp. 32, 46, 50. [27] Fauna und Flora des Golfes von Neapel. Die Actinien. 1884.