This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(1282). A very remarkable feature in the history of these animals is that many species are found swimming together in long chains, apparently adhering to each other by little. suckers, but without organic connexion; and, what is still more strange, it would appear, from the observations of M. de Chamisso1, that such aggregated animals give birth to insulated individuals of very different appearance, which in their turn reproduce concatenated forms resembling their progenitors; so that the alternate generations are quite dissimilar both in conformation and habits.
(1283). The observations of Chamisso have in later times been substantiated and carried out by the researches of Krohn, Steenstrup, Eschricht, Milne-Edwards, and others; and the phenomena connected with the process are so interesting that it will be necessary to lay before the reader a brief abstract of the result of their labours.
(1284). The Salpae are all viviparous, and each species is propagated by an alternate succession of generations most dissimilar from each other in their forms, habits, and mode of increase. The concatenated Salpae produce but a single egg apiece, which is distinctly visible in the interior of their transparent bodies; they seem, moreover, to be bisexual, having, as it would appear, two generative functions to perform - the one to produce a new being, the other to fecundate a future generation of animals similar in all respects to themselves. But whilst the isolated Salpians are thus produced from eggs, their progeny are produced by a process of gemmation, springing like buds from the surface of a most remarkable organ, the stolon prolifer, the existence of which is to be detected in the isolated Salpae even while contained in the body of their concatenated parent; it then appears a slender filament, derived by one extremity immediately from the heart of the embryo Tunicary; after birth, however, its growth increases apace in proportion to the development of the continual succession of progeny to which it gives origin.
The reason of the immediate connexion between the "stolon proliferum" and the maternal heart appears to be this, that the newly-formed offspring being entirely dependent for support upon the blood of the parent, it is so situated in order to secure a free supply of the vital fluid, which is thus injected into its vessels immediately by the heart's action. Two vessels traverse it throughout its entire length, one derived from the anterior extremity of the maternal heart, and the other from its opposite end; so that the blood supplied to one of these vessels by the contraction of the heart is returned by the other; and when the contractions of the heart become reversed, as we have seen is continually the case by this arrangement, the circulation in these vessels is readily adapted to the change. It is from the surface of the stolon that the generation of concatenated Salpae sprout, in two parallel rows, appearing in rapid succession like so many little buds, which, as their growth advances, gradually assume a similar form and structure; and as successive groups become mature, detaching themselves from the nidus where they had their birth, they swim away united in long chains, the links of which are joined together after the fashion of the species.
On examining one of these chains of concatenate Salpae, the individuals composing it are found to be united to each, not by any organic coalescence, but by special organs and facets of attachment, frequently, but improperly, described as suckers, the position of which varies in different species in accordance with their mode of aggregation. It would appear, from the observations of M. Krohn, that the concatenated Salpae cannot spontaneously detach themselves from each other, and that, when individuals are met with swimming free, their separation from the chain is always to be ascribed to accidental violence; he even thinks that concatenation is so essential to the existence of the animals that they soon perish if separated from the rest.
* For excellent drawings representing the anatomy of various Salpce, the reader is referred to the Descriptive and Illustrated Catalogue of the Physiological Series of Comparative Anatomy contained in the Museum of the Royal College of Surgeons of England, vol. i. plates 6 & 7. 1 Dissert, de Salpa, Berlin, 1830.
(1285). The last families of Tuntcata which we have to notice would seem to constitute a connecting link between the Ascldians and the Polyzoa, which latter in many points of their anatomy they much resemble. These animals generally are exceedingly minute, and individually present an organization analogous to that of Ascidians. At first it would appear that they are detached from each other, and, like Salpae, are endowed with a power of locomotion; but subsequently they become aggregated in groups, either incrusting foreign bodies, or else, uniting together to form a mass of definite shape, they seem to enjoy, to a certain extent, a community of action. They are arranged by Cuvier* in three principal groups, distinguished by the following characters. In the first (Botryllus1), the little bodies of the individual animals are ovoid; but they fix themselves upon the exterior of sea-weed or other substances in regular bunches, consisting of ten or twelve, arranged like the rays of a star around a common centre. The branchial orifices in such are all placed around the circumference of the star, while the excretory apertures open into a common cavity in the centre.
If the external orifice is irritated, the animal to which it belongs alone contracts; but if the centre be touched, they all shrink at once.
(1286). In Pyrosoma2, the second family, the animals are aggregated together in great numbers, so as to form a hollow cylinder, open at one end, but closed at the opposite, which swims in the sea by the combined contractions and dilatations of all the individuals composing it. The branchial sacs here open upon the exterior of the cylinder, while the anal orifices are in its internal cavity. Thus a Pyrosoma might be described as consisting of a great number of stars of Botrylli piled one above the other, the whole mass remaining free and capable of locomotion. Many of these moving aggregations of Tunicata emit in the dark a most brilliant phosphorescent light, whence the derivation of the name by which they are distinguished.
(1287). In all other forms of these aggregated Mollusca, which are designated by the general name of Polyclinum§, as in ordinary Asci-dians, the anus and branchial orifices are approximated, and placed at the same extremity of the body. They are all fixed; some spreading like fleshy crusts over submarine substances, others forming conical or globular masses, or occasionally so grouped as to produce an expanded disk resembling a flower or an Actinia; but, whatever the general arrangement of the common mass, it is composed of numerous associated individuals, every one of them corresponding more or less closely, as regards their internal structure, with the description above given of the organization of Salpae and Ascidians.
* Regne Animal, vol. iii. p. 168.
A bunch of grapes.