Part 49

The _Basis_ is membranous, and more or less convex. This convexity is rendered possible by the support afforded by the basal edges of the six midribs. The so-called epidermis or membrane which covers the shell, is prolonged in six flaps under the midribs; and hence the proper basal membrane has six deep indentations, receiving the above flaps. The cementing apparatus is constructed on the same simple type as in Coronula, as has been described in the Introduction.

_Scuta and Terga._--These are narrow and long, stretching the whole length of the orifice of the shell. The two valves touch each other, but are not properly articulated together: their upper surfaces scale off in layers: they resemble each other very closely in shape (fig. 1 _c_), being simply oblong. The membrane covering the valves supports some very minute spines, 2/1000th of an inch in length. When a portion of valve is dissolved in acid it is seen to be penetrated by very numerous tubuli. The opercular membrane is rather thick: in _P. decorata_ I found it occasionally double; in this species it supports some very minute spines; it is attached about one third down the sheath, of which the upper part is not marked (as in Balanus) by regular concentric rings. The aperture between the valves, leading into the sack, has protuberant lips, as in Coronula and Tubicinella. The normal five bundles of muscles surrounding the sack, differently from in the two genera just mentioned, do not thin out and spread out downwards, but retaining their full thickness, reach the basal membrane, to which they are attached.

_Mouth_: the parts present no difference from those in Coronula; between the outer maxillæ we have the same small flattened mentum-like projection; and in the mandibles, the same small intermediate teeth between the second and third, and between the third and fourth main teeth, as in that genus. The main teeth here are more plainly double laterally. There are teeth on each side of the central notch of the labrum. The cirri differ little from those of Coronula, but the segments are not nearly so much flattened and broad, and the second and third pairs are not so short. The segments of the sixth pair support four pairs of main spines, with a few minute intermediate spines, but with no intermediate tuft.

_Branchiæ_: these consist on each side (at least in _P. decorata_) of a double fold, much less plicated than in Coronula; the fold nearest to the animal's body is rounder, and not quite so large as the outer fold.

_Range and Habits._--The _P. bissexlobata_ ranges from the Mediterranean and west coast of Africa to the West Indies, to the north-east coast of Australia, and, judging from the name given by Chenu, to California: the second species, _P. decorata_, inhabits the Pacific Ocean; and the third imperfectly known species, was deeply buried in the skin of a sea-snake, off Borneo. The _P. bissexlobata_ is attached both to turtles and to the skin of the manatee: some specimens in the British Museum, attached to the latter, from the west coast of Africa, were entirely buried in the skin, with the exception of the operculum. The specimens of _P. decorata_, which I obtained at the Galapagos Archipelago, were buried about half their depth in the softer parts of the skin of the green turtle. I do not doubt that the imbedment is effected in the same manner as in the case of Chelonobia.

_Affinities._--This genus, in general structure and habits, approaches closely to Coronula; but it differs from all the species of that genus in the following respects, which appears to me fully to justify its generic separation:--The outline is often oval, instead of circular; the compartments are not of equal sizes and shapes; each compartment has here only one inward furrow or fold, and this has its sides pressed closely together, and is encased, but not obliterated, by the sheath; the rostrum is not quite medial; and the radii are narrow and nearly solid. The opercular valves, again, are here more developed, and their microscopical structure is different. The opercular membrane is less constantly double; and, what is much more important, the muscles of the sack are not here spread out, and show no tendency to become rudimentary and lose their transverse striæ. The cirri differ only in not being so broad and flattened. The branchiæ are not quite so much developed. Finally, I may add, that in several of the above respects in which Platylepas differs from Coronula, it approaches ordinary sessile cirripedes.

1. PLATYLEPAS BISSEXLOBATA. Pl. 17, fig. 1 _a_-1 _d_.

CORONULA BISSEXLOBATA. _De Blainville._ Dict. des Sciences Naturelles, tom. 32, (1824), Tab. 117, fig. 1.

PLATYLEPAS PULCHRA. _J. E. Gray_ (!). Annals of Philosophy, (new series), vol. 10, (1825).

COLUMELLINA BISSEXLOBATA. _Bivona_ (fide _Philippi_). Nuovi generi di Mollusch. (1832), Tab. 3, fig. 1.[126]

CORONULA CALIFORNIENSIS. _Chenu_ (!). Illust. Conch., Tab. 1, fig. 4.

[126] This memoir was published in the 'Effemeridi Scientifiche e Litt. per la Sicilia,' according to the 'Bibliographia Zoologiæ et Geologiæ,' by Agassiz and Strickland.

_Shell with the transverse lines of growth conspicuous: parietes permeated by pores; sheath descending barely half-way down the parietes._

_Hab._--Mediterranean, attached to turtles. River Gambia, attached to manatee. Honduras, attached to manatee. Moreton Bay, lat. 27° S., Australia, apparently attached to the dugong of that coast. California(?) Mus. Brit., Stutchbury, and Cuming.

_General Appearance._--Shell generally much depressed, and broadly oval or circular; sometimes steeply conical. Orifice oval, generally not large. Surface rather plainly marked by closely approximate lines of growth, which with the lobed outline gives to the whole an elegantly sculptured appearance: occasionally the longitudinal ridges formed by the parietal septa are distinct. Basal diameter of largest specimen three quarters of an inch.

_Structure of Shell._--I have nothing material to add to the generic description. The midribs are not so prominent as in the following species, and hence the basal membrane is less convex. The origin of the midrib as a fold is very plain. The sheath descends barely half-way down the walls, and is a little hollow on its under margin, on each side of the midrib. Beneath the sheath the parietes are finely ribbed (fig. 1 _d_), but to a variable degree. I may here remark, that in the specimens taken from manatee, on the coast of Africa and at Honduras, the internal ribs extended further up and were plainer, and the opercular valves seemed to be a little narrower than in the other specimens, so that I at first suspected that they were specifically distinct, but I could make out no other than these small and variable points of difference.

_Scuta_: oblong (fig. 1 _c_), about twice as long as broad, with the rostral end rounded, rather narrower than the other end, and curled a little inwards. _Terga_, of nearly the same shape and nearly as long as the scuta: the carinal end is rather more pointed than the scutal end of the valve, and when viewed internally, the growing surface of this end is seen to be bluntly pointed. In both valves the upper layers of shell usually scale off.

_Mouth and Cirri._ The only differences, compared with the following species, that I perceived, were that the hairs at the end of the palpi were here rather longer, and the fourth tooth in the mandibles rather less distinct. The rami of the first cirrus are very unequal in length.

2. PLATYLEPAS DECORATA. Pl. 17, fig. 2 _a_-2 _b_.

_Shell with fine longitudinal ridges, ornamented in the lower part by minute beads; parietes not porose: basal membrane equalling in convexity the shell._

_Hab._--Pacific Ocean; Galapagos Archipelago; Lord Hood's Island, Low Archipelago; Mus. Cuming, Darwin.

_General Appearance and Structure._--Shell oval, with the orifice large. The walls are thick, and of less height from top to base than in the last species. The surface (and this is the chief external character) is marked by fine longitudinal ridges, each of which, when closely examined, is found to be double (fig. 2 _b_), and near the basis, where not abraded, to be ornamented with minute beads on each side: these beads are largest on those ridges which border the sutures. The parietes are not permeated by pores; on their inner surfaces there are a few rather prominent but short ridges, instead of the many finer ridges, as in _P. bissexlobata_. The sheath is of great thickness; in young specimens layers scale off its summit, all round the shell, as in Tubicinella; it is only slightly hollowed out at its lower margin; it descends more than half-way down the walls. The midribs, formed by the folded walls, are more prominent than in the last species, and descend lower down; hence the basal membrane is more convex than in the last species, for it projects downwards as much as the shell projects upwards. In medium-sized specimens, the midribs project inwards to a distance exceeding half the breadth of a compartment. A less proportional length of the midrib, in an inward direction, is formed by the inward folding of the wall, and a greater length by the thickening of its inner longitudinal margin, than in the last species. The basal edge of the inner end of the midrib is smooth.

_Dimensions._--The average size of the numerous specimens which I obtained at the Galapagos Islands was about .2 of an inch in diameter, and I found none larger; but Mr. Cuming's specimens from Lord Hood's Island are half an inch in diameter.

_Opercular Valves._--These closely resemble those of the former species, with the exception that the carinal end of the growing or under surface of the Tergum is much squarer.

_Cirri._--The only difference which I could perceive was, that the rami of the first pair were not quite so unequal in length.

_Species Dubiæ._

3. PLATYLEPAS ----?

_Hab._--Imbedded in the skin of a sea-snake, taken off Borneo.

I am indebted to Dr. Gray for a single specimen of this supposed species, but as it is very young and imperfect, wanting the opercular valves and cirri, I do not choose to name it. The shell presents all the usual characters of the genus; the rostrum, I may remark, being pushed to the left side. The parietes are permeated by pores of considerable size, which shows that the species is distinct from _P. decorata_. On the inner basal surfaces of the walls, there are two or three very distinct ridges on each side of the midrib; and this fact, together with the size of the parietal pores, makes me suspect that it is not an immature specimen of _P. bissexlobata_.

10. _Genus_--TUBICINELLA. Pl. 17, fig. 3 _a_-3 _c_.

TUBICINELLA. _Lamarck._ Annales du Museum, tom. 1 (1802), Tab. 30, fig. 1.

CORONULA. _De Blainville._ Dict. des Sciences Naturelles, (1824).

_Compartments six, of equal sizes; shell sub-cylindrical, wider at the top than at the basis, belted by several large transverse ridges._

_Hab._--Southern Pacific Ocean, Western South America, New South Wales, Cape of Good Hope; imbedded in whales, and often associated with _Coronula balænaris_.

This genus is closely allied to Coronula, and perhaps De Blainville was right in uniting them. But Coronula, as it now stands, is so natural a genus, that it seems a pity that a form so entirely different in general aspect, as Tubicinella, should be forced into it. The main difference between these genera consists in the walls being here not folded, in the simpler radii, and in the general shape of the shell and of the included animal's body; but there are many other minor points of difference. The most novel character in Tubicinella consists in the shell being lined almost close down to the basis by the opercular membrane. In the opercular membrane thus forming a long tube, and in the general shape of the animal's body, we shall presently see that Tubicinella is closely related to Xenobalanus. In numerous other respects Tubicinella is almost equally allied to the latter genus, to Platylepas, and to Coronula. Finally, several points of structure indicate that Tubicinella may be considered as a Coronula, with the shell much simplified in structure.

1. TUBICINELLA TRACHEALIS. Pl. 17, fig. 3 _a_-3 _c_.

LEPAS TRACHEALIS. _Shaw._ Nat. Miscell. (1789-1813), vol. 17 (1806?) tab. 726.

---- TRACHEÆFORMIS. _Wood._ General Conch. (1815), tab. 4, fig. 1-3.

TUBICINELLA TRACHEALIS. _J. E. Gray._ Annals of Philosophy, (new series), vol. 10, (1825.)

---- MAJOR ET MINUS. _Lamarck._[127] Annales du Mus. Nat., tom. 1 (1802), Tab. 30, fig. 1-2.

---- BALÆNARUM. _Lamarck._ Animaux sans Vertèbres, (1818).

---- ---- _Chenu._ Illust. Conch. (Plate).

---- ---- _Sowerby._ Genera of Recent and Fossil Shells (Plate).

---- LAMARCKII. _Leach._ Encyclop. Brit. Suppl., vol. 3 (1824), Pl. 57.

CORONULA TUBICINELLA. _De Blainville._ Dict. des Sciences Nat., tom. 32, Pl. 117, fig. 5 (1824).

[127] It may be observed that I have here broken through the great law of priority; for it appears to me too grossly incorrect to retain the specific name either of _major_ or _minus_ in a genus including a single species. Lamarck himself seems to have been of this opinion, by giving, in 1818, the new specific name of _balænarum_; but Shaw's name of _trachealis_ has the clear right to priority, if _major_ or _minus_ be rejected.

_General Appearance._--Shell elongated, sub-cylindrical, with the upper end rather wider than the lower, and therefore widening in a direction the reverse of that usual with sessile cirripedes. The shell is often a little bent to one or the other side: it is surrounded by from two or three to about ten very prominent, strong, blunt ridges or belts, placed at rather irregular distances from each other. The surface is finely striated longitudinally. The six compartments are of nearly equal sizes and shapes. In full-grown specimens the parietes are not wider at the base, and often they are even a little narrower, than at the summit of the shell: in young specimens the parietes do widen a little downwards. The radii are narrow; but in young specimens they are proportionably much broader (Pl. 17, fig. 3 _b_) than in old specimens. The whole compartment, including the radius and wall, is always a little wider at the summit than at the base, in accordance with the shape of the whole shell. The operculum consists of four nearly equal-sized, similar valves, projecting above the upper end of the shell, which is always broken and jagged: the valves are united to the sheath by a very thick, much folded membrane. The aperture leading into the sack is bordered by very prominent lips, projecting above the opercular valves; the latter have their upper layers always scaled off. The shell is imbedded in the whale's skin up to the level of its operculum. The largest specimen which I have seen was barely one inch (.95) in diameter at the summit, and 1.5 in length; the longest specimen which I have seen scarcely attained a length of one inch and three quarters.

_Structure of the Shell._--The parietes are thin, and if the sheath (which extends to near the basis) be removed, they are rendered extremely thin. They are formed by an outer and inner lamina, united by fine longitudinal septa, projecting at the basis beyond the laminæ. The pores thus produced (which in a transverse section are oblong in outline), run up to the summit of the shell, and are not filled up by shelly matter; but I presume that the included tubular threads of corium are protected, at the broken upper end of the shell, by transverse membranous septa. The outer lamina of shell, as in Coronula, is formed, though obscurely, by the union of ledges projecting from the longitudinal septa. The circular prominent belts, surrounding the shell, are formed by the longitudinal septa, at certain, irregular and rather distant periods, growing outwards; the wall at each belt being increased to nearly twice its thickness in other parts. At each belt the threads of corium within the parietal pores lend off minute branches to supply the thickened wall. These belts, which continuously surround the shell, correspond (as is best seen in young specimens), with the little knobs or beads, which, in Coronula (Pl. 16, fig. 4), rise separately, and not quite regularly, on the longitudinal parietal septa, and which, I believe, are formed at every successive period of growth; here they are much larger, stand in straight transverse rows, become confluent, and are formed only at occasional intervals. The whole external surface of the shell is covered by membrane, stronger and more persistent than is usual with most cirripedes.

Internally the sheath extends almost to the basis of the much elongated shell, and terminates in a slight shoulder: it is divided as in common Balanidæ, and differently from in Coronula, into zones of growth, but these are very broad; at the upper end of the shell, which, as will hereafter be explained, is always breaking away, the sheath readily yields along the oblique planes, which separate the zones of growth and dip outwards: a similar but less strongly marked structure occurs in Platylepas, and in no other genus. We shall presently see that the sheath presents a much more anomalous character, in being lined down to its basal edge by the innermost and last-formed layers of the opercular membrane.

_Radii._--The radii are narrow. The belts which surround the shell are prolonged, with slightly diminished prominence, across the radii, their formation being simply due to the radii being here thicker than in other parts. It can be seen more plainly in Tubicinella, than in other Balanidæ, that the membrane externally investing the shell, splits along the radii during the diametric growth of the shell, and is continually repaired and added to along these lines by new longitudinal slips of membrane. The radius consists (as usual) of an inner and outer lamina, which latter does not extend quite to the line of suture--a slightly gaping fissure being thus left. The two laminæ are connected by septa, which are not denticulated, but near the outer lamina bi- or tri-furcate, and the ends of the branches thus formed spread out, forming a sort of outer scalloped lamina, in advance of the true outer lamina. The fine threads of corium running between these septa, do not spring, as in all common cirripedes, from a fillet of corium occupying the actual suture, but from two nearly circular threads of corium occupying two tubes, which run along the line of junction between the radius and the compartment whence it springs. In Coronula alone we have a nearly similar structure; for the fine threads of corium occupying the _proper_ radius, spring from a _single_ very minute tube (Pl. 16, fig. 7, _d′_), occupying the same position with the _two_ tubes in Tubicinella. I may further add, that the structure of the proper radius in Coronula is precisely the same as here just described, but being on so very minute a scale, I did not there describe it so carefully as I have here done.

_Alæ_: these are only remarkable from their extreme thinness; for they are not thicker than the inner lamina of the radius. Their sutural edges are quite smooth. Forming part of the sheath, they extend down close to the basis of the shell; where, instead of, as in general, ending abruptly in a rectangular shoulder, they slope off into their own compartment.

_Basis._--The basal membrane is complicated, owing to the shell, when full-grown, barely, or not at all, increasing in diameter, and, in consequence, membrane after membrane, each with its own cement-ducts attached to it, are thrown down one nearly over the other. In the Introduction (p. 143--Pl. 28, fig. 3), I have fully described the cementing apparatus, which is very curious from one of the ducts always having a loop with two spurs projecting from it. The basal membrane does not equal in diameter the base of the shell, for the membrane externally covering the walls is inflected inwards all round for a considerable width, and is then united to the basal membrane: in Coronula, the basal membrane extends only under the internal cavity of the shell, and not under the folded walls, and therefore presents a somewhat analogous structure.

_Opercular Valves._--The scuta and terga are nearly of the same size and shape: they are mitre-formed, and higher than broad. They do not fill up the orifice of the shell. The scutum is a little larger than the tergum, and rather less symmetrical, the rostral corner of the valve being a little produced. There is no hollow or crest for the adductor muscle, which is small. In the tergum there is no trace of a spur. The two valves are not articulated together, but standing close to each other are united, as well as the scutum to the scutum, and tergum to the tergum, by thick, brown, tough, yet soft membrane, in layers continuous with, but differing in appearance from, the surrounding opercular membrane. The layers of shell, forming the valves, are thick, and only the three or four lower layers are usually preserved, the upper ones having symmetrically scaled off, leaving snow-white surfaces. Owing to the thickness of the successive shelly layers, and to the circumstance of each new layer being but very little larger than the last, the scaling off of the old upper layers is a quite necessary process; for otherwise the orifice into the sack would have been encumbered and almost closed by four long, slightly tapering points, prolonged upwards from the basal layers that form the four existing valves. The same scaling off process takes place in Platylepas, and amongst pedunculated cirripedes in Lithotrya. Microscopical examination does not exhibit any fine spines on the membrane investing the valves, or any tubuli in the shelly layers after their dissolution in acid: in this respect the valves resemble those of Coronula. The summits of the valves project freely for about a third of their own height, above the level of the membrane by which they are surrounded. The orifice leading into the sack is bordered by very protuberant lips, standing up even considerably above the upper freely projecting portions of the valves.