Fig. 287.—Aplidiopsis jordani Ritter, a compound Ascidian. Lukanin Beach, Pribilof Islands. (After Ritter.)
The Salpas abound in the warm seas, the chains often covering the water for miles. They are perfectly transparent, and the chains are often more than a foot in length. In Doliolum the body is barrel-shaped and the gills are less modified than in Salpa. The alternation of generations in this genus is still more complicated than in Salpa, for here we have not only a sexual and a non-sexual generation, the individuals of which differ from each other, but there is further a differentiation among the asexually produced individuals themselves; so that we have in all three instead of two sorts of animals in the complete life cycle. Besides the proliferating stolon situated on the ventral side, the bud-producing individual possesses a dorsal process larger than the stolon proper. The buds become completely severed from the true stolon at an early stage and actually crawl along the side of the parent up to the dorsal process, upon which they arrange themselves in three rows, two lateral and one median. The buds of the lateral rows become nutritive and respiratory zooids, while those of the median row, ultimately at least, give rise in turn to the egg-producing individuals.
Origin of Tunicates.—There can be little doubt that the Tunicata form an offshoot from the primitive Chordate stock, and the structure of their larva in connection with that of the lancelet throws a large light on the nature of their common parents. "We may conclude," says Dr. Arthur Willey, "that the proximate ancestor of the Vertebrates was a free-swimming animal intermediate in organization between an Ascidian tadpole and Amphioxus, possessing the dorsal mouth, hypophysis, and restricted notochord of the former and the myotomes, cœlomic epithelium, and straight alimentary canal of the latter. The ultimate or primordial ancestor of the Vertebrates would, on the contrary, be a worm-like animal whose organization was approximately on a level with that of the bilateral ancestors of the Echinoderms."
Fig. 288.—Adult Tunicate of the group Larvacea, Oikopleura. Family Appendiculariidæ. (After Fol, per Parker & Haswell.)
Degeneration of Tunicates.—There is no question, furthermore, Professor Ritter observes, "that most of the group has undergone great degeneration in its evolutionary course. Just what the starting-point was, however, is a matter on which there is considerable difference of opinion among authorities. According to one view, particularly championed by Professor W. K. Brooks, Appendicularia is very near the ancestral form. The ancestor was consequently a small, marine, free-swimming creature. From this ancestor the Ascidiacea were evolved largely through the influence of the attached habit of life, and the tadpole stage in their development is a recapitulation of the ancestral form, just as the tadpole stage in the frog's life is a repetition of the fish ancestry of the frog.
"According to the most common view Appendicularia is not an ancestral form at all, but is the tadpole stage of the Ascidiacea that has failed to undergo metamorphosis and has become sexually mature in the larval condition, as the larva of certain Amphibians and insects are known to never pass into the adult state but reproduce their kind sexually in the larval condition. By this view the tadpole of such Ascidian as Ciona, for example, represents more closely the common ancestor of the group than does any other form we know. This view is especially defended by Professor K. Heider and Dr. Arthur Willey."