Appendisc. : Tunicata. 



537 



Appendix to the Vektebeata. 

 Tunicata {Sea-Squirts). 



The Tunicata are a small group of marine animals, which were 

 formerly usually regarded as Mollusca, or placed with some other 

 division of the Invertebrata ; only recently has it been demonstrated 

 that they are most nearly allied to the Vertebrata, a relationship 

 which is made specially clear by a consideration of their ontogeny. 

 In particular it has been shown that in early life at least they agree 

 with the Vertebrata in the possession of a notochord, and in the 

 position of the central nervous system, both fundamental 

 points. In spite of this, they are not, however, incorporated with the 

 Vertebrates, but treated of in an appendix, because the majority 

 undergo so peculiar a metamorphosis that the Vertebrate characters 



Fig. 425. A diagram of one of the Appendioularia, viewed from the side, 

 stretched out straight. B ditto of an Ascidian larva, a anus, ch chorda, g 

 branchial ohamher, m mouth, n train, n' nerve cord, t gut. — Orig. 



have completely disappeared in the adult, which has received an 

 entirely different impress : thus it is more convenient to consider 

 them separately. 



It may also be noted here that the Tunicata, like the true Vertebrata, do not 

 exhibit definite affinities with any of the Invei-tebrata. 



Insight into the characteristics of this group is best attained by a 

 separate consideration of its various sub-divisions belonging to it. 

 The following general characteristics may, however, be noticed : the 

 skeleton is at best only represented by the aotochord, the 

 nervous system is feebly developed, so also are the sense 

 organs. The Tunicata are hermaphrodite, ovaries and testes 

 are continued directly into their ducts. Eepr eduction by budding 

 frequently occurs. 



The Appendioularia possess the simplest and most easily com- 

 prehended organisation. They are tiny, transparent, free-swimming, 

 marine forms, with some resemblance to tadpoles. The body is 



