a, Rostrum; b, Rostro-lateral, c, Lateral, d, Carino-lateral compartment; e, Carina.
Horizontal sections through the Shells of the principal genera of [Balanidæ], showing the arrangement of the Compartments. Genera 4, 5, and 6 belong to the [Chthamalinæ]; 7 and 8 to the [Balaninæ].
Number and Arrangement of the Compartments.—I have already stated that the shell, in every one of the [Balanidæ], consists of eight, six, or four compartments, or of all fused together into a single piece; and that the compartments themselves are all constructed on the three simple patterns of which woodcuts (figs. [1], [2], [3]) have been given. They are arranged in a certain definite order. The type arrangement is found amongst the [Chthamalinæ], as might have been expected, inasmuch as this sub-family is so closely related to the ancient genera Pollicipes and Scalpellum, whence all the Thoracic Cirripedia may be said to radiate. In [Octomeris] (fig. [4]) the type-arrangement of the compartments, eight in number, is well shown; the rostrum and carina resemble each other, and have alæ on both sides, and therefore are overlapped on both sides: the rostro-lateral compartments have radii on both sides, and therefore overlap the adjoining compartments on both sides; the lateral and carino-lateral compartments have radii on their carinal, and alæ on their rostral sides; and therefore overlap on one side, and are overlapped on the other side. Now the shell of every other sessile Cirripede differs, I believe, from that of [Octomeris], only in the fusion together or abortion of some of the eight normal compartments: in one genus, however, [Catophragmus], outer whorls of small compartments, arranged like the lower valves in the capitulum of Pollicipes, are superadded. The genus [Chthamalus] (fig. [5]) differs from [Octomeris] only in the carino-lateral compartments being aborted, (as will presently be discussed), and hence has six compartments. [Chamæsipho] (fig. [6]) differs from [Chthamalus] only in the rostro-lateral and lateral compartments being fused together; and hence has only four compartments. In [Balanus] (fig. [7]) and the whole sub-family of the [Balaninæ], the rostrum is compounded of the true rostrum, as seen in the type [Octomeris], and of the two rostro-lateral compartments; hence this compounded rostrum has radii instead of alæ on both sides, and there are only six compartments. [Tetraclita] (fig. [8]) and [Elminius] differ from [Balanus] only in having the carino-lateral compartments absent, and probably aborted; hence there are only four compartments. Lastly, in [Pyrgoma], all the compartments are blended together into a single piece.
In Pollicipes, the old type-form of the whole order, and in Scalpellum, we have four valves, (answering to the operculum), surrounding the aperture leading into the sack, and the valves below are arranged in successive whorls, with a strong tendency to alternation. For, the rostrum alternates with, that is faces the interval between, the two scuta; the carina alternates with the two terga; and the upper lateral valves alternate with the scutum and tergum on each side. These four valves, namely, the carina and rostrum, which resemble each other in structure, and the pair of upper latera, which are larger than the other lateral valves, together form the uppermost whorl, or that beneath the scuta and terga. In the next whorl we have the rostro- and carino-lateral valves, alternating with those above them; and beneath them there are generally other valves, which decrease in size and display the same tendency to alternation. The valves here just specified, namely, the rostrum, carina, and three pairs of lateral valves, in the Lepadidæ, are so much larger, and are so much more commonly present, than the other valves of the capitulum, that to them alone I affixed special names. Now if amongst sessile Cirripedes we look to that genus, viz., [Catophragmus], which comes in its whole structure the nearest to Pollicipes, one of the Lepadidæ, we find (as in fig. [4]), firstly, a rostrum and carina resembling each other, and a pair of lateral compartments, larger than the other lateral pairs; these four valves alternating with the opercular valves: and, secondly, we find, but forming part of the same whorl, a pair of rostro-lateral and a pair of carino-lateral compartments, which, just as in Pollicipes, are larger than the exterior and lower valves. These lower little valves, I may remark, decrease in size in the successive whorls, and tend to alternate in position, just as in Pollicipes. Observing these several striking points of correspondence in the valves, (and indeed in the whole structure), of [Catophragmus] and Pollicipes, one is strongly inclined to suspect that in [Catophragmus], and therefore in [Octomeris] and other sessile Cirripedes, although the rostro- and carino-lateral compartments appear to lie in the same whorl with the rostrum, carina, and large lateral compartments, yet that they really belong, as in Pollicipes and Scalpellum, to a lower whorl. Now if a very young shell of [Balanus], immediately after the metamorphosis, be examined, the carino-lateral compartments will be found not to have been developed; they first appear after two or three zones of growth have been added to the other compartments; bearing in mind that in Pollicipes and in [Catophragmus] the lower whorls are added successively during growth, we find in this fact strong confirmation of the view that the carino-lateral compartments normally belong to a whorl beneath that including the rostrum, carina, and lateral compartments. Whether the rostro-lateral, like the carino-lateral compartments, are developed subsequently to the others, I have had no opportunity of ascertaining, and therefore cannot confirm the above analogical conclusion, namely, that they, also, belong to a lower whorl.
In the sub-family [Balaninæ], which includes [Balanus] (woodcut [7]), and [Tetraclita] (woodcut [8]), the shell is characterised by not having rostro-lateral compartments, and by the rostrum having radii: now in [Pachylasma giganteum], which undoubtedly belongs to the sub-family [Chthamalinæ], at a very early age the rostro-lateral compartments become blended with the true rostrum, making a compound rostrum, exactly like the rostrum in the [Balaninæ]; distinct evidence of a similar fusion is retained throughout life (Pl. [15], fig. [1]) in all three species of [Chelonobia], which is undoubtedly a member of the [Balaninæ]. Hence, I think, we may conclude that in all the genera of the [Balaninæ] the rostro-lateral compartments are probably not aborted, but are blended with a normal rostrum (resembling that in woodcuts [4], [5], [6]), making together a compound rostrum furnished with radii: it must, however, be observed that I could not detect any actual evidence of this fusion in [Balanus], even immediately after the metamorphosis. In [Chamæsipho] (woodcut [6]), either the rostro-lateral compartments attain a most unusual breadth, or, as is more probable, they have become confluent with the lateral compartments, which in the Lepadidæ seem to be the most persistent of all the lateral valves. In such genera as [Tetraclita] and [Chthamalus], in which the carino-lateral compartments are absent, they may be fused with the lateral compartments or with the carina; but seeing that they are normally developed later than the other valves, it appears to be the simplest theory to assume, until the contrary be proved, that they are aborted. Finally, the somewhat unexpected conclusion that the shell (not including the operculum) of sessile Cirripedes normally consists of eight valves, four belonging to an upper whorl, and four to a lower whorl, all forced into a single ring, and often more or less fused together, though not strictly proved, is rendered highly probable. I will only further add, that the Basis perhaps represents several whorls of the small valves or scales on the peduncle of Pollicipes, fused together; the comparison of the basis with the calcareous cup, forming the lowest portion of the peduncle in Lithotrya, which has been made by some authors, I do not think is very accurate, as the cup in Lithotrya seems to have a special relation to the boring habits of that genus.
Fig. 9.
Basal edge of wall of compartment in [Balanus tintinnabulum]; a, a, outer lamina; b, b, inner lamina; c, c, longitudinal septa uniting the inner and outer laminæ with their ends denticulated.