Fig. 7.
Very young Ascidian with only two gill-slits.
(Figs. 6, 7, from Lankester’s ‘Degeneration.’)
This wonderful metamorphosis presents a striking example of DEGENERATION resulting from the adoption of a fixed mode of life. The active free-swimming larva with its brain, eye, hearing organ, and muscular tail becomes transformed into a comparatively inert sac.
Fig. 8.
Tadpole of Frog and Ascidian. Surface view. (Lankester’s ‘Degeneration.’)
Fig. 9.
Tadpole of Frog and Ascidian. Diagram representing the chief internal organs. (Lankester’s ‘Degeneration. A chapter in Darwinism.’)
The tadpole of an Ascidian resembles that of a frog (Figs. 8, 9), not merely superficially, but also in its general structure and mode of development. The Tunicata are now generally regarded as a degenerate offshoot from the ancestral stock of the Vertebrata, in that the larva possesses a skeletal rod (rudimentary backbone) separating the dorsally situated nerve-tube (cerebrospinal axis) from the ventrally situated intestinal tube, the existence of the cerebral eye in the Ascidian tadpole further tending to confirm the truth of this theory. Apart from a knowledge of the course of their development, Tunicata would have been classed among the Invertebrata, but the structure of the larva clearly reveals the affinities of the group to the backboned animals.
Ascidia mentula belongs to the group of Simple Ascidians which are all fixed, and are either solitary or joined into colonies in which each individual or ascidiozooid has a distinct test of its own. In the Compound Ascidians, which form colonies by budding, the ascidiozooids are buried in a common investing mass and have no separate tests. In a third group, the Salpa-like Ascidians, the ascidiozooids are united to form free-swimming colonies shaped like hollow cylinders open at one end. The above three groups belong to one great Order—the Ascidiacea. A second Order, Thaliacea, includes the free-swimming Salpa and Doliolum, which exhibit alternation of generations in their life history. A third Order Larvacea, includes very minute free-swimming forms which possess a tail in the adult stage. There are sixteen families of Tunicata.
The following is a tabular view of Prof. Herdman’s classifications:—