Appendix : Tunicata. 539 



colourless. The central nervous system is reduced to a 

 nerve ganglion situated between the mouth and the cloacal aperture. 

 Ductless vesicles varying in size perform the function of excretory 

 ■organs; they excrete hard particles, in which the presence of uric 

 acid has been demonstrated. The reproductive organs (an 

 •ovary and a testis) open into the peribranchial cavity close to the 

 anus. The body is entirely covered with a thick gelatinous or leathery 

 ■coat, the "mantle," a product of the epidermis.* 



The difference between the Appendicularia and the adult Ascidian 

 is thus very great. But a comparison of the larval forms reveuls 

 quite other relations ; there is a close agreement in almost every 

 respect. The Ascidian larvae are still more like Tadpoles than are 

 the Appendicularia ; they have a rounded trunk, and a long com- 

 pressed tail with a notochord, which extends a short distance 

 into the trunk. Dorsal to the notochord lies the central nervous 

 .system, which extends along the whole tail, and has a swelling, the 

 brain, anteriorly; in connection with the latter, there is an eye and 

 .an organ which is regarded as auditory ; both are remarkable in that 

 they lie within the cavity of the brain, and are specially developed 

 portions of its wall. There is as yet no peribranchial cavity, but 

 two simple gill slits lead from the pharynx to the surface. 

 As may be seen, therefore, there is very great resemblance between 

 these larvffi and the Appendicularia. Before long, however, the larva 

 attaches itself, the tail dwindles; the notochord, the sense organs, 

 ■etc., degenerate ; and the animal gradually attains to the very aberrant 

 form of the adult Ascidian. 



It is easy to see how closely tlie type of structure of ttis larva corresponds 

 with that of the Yertebrata (as regards the position of the nervous system, the 

 notochord, and the alimentary canal) ; the relation is stUl more obvious than in 

 the adult Appendicularia, where the rotation of the tail, etc., to some extent, 

 masks the conformity. It must also be noticed here that the notochord arises in 

 exactly the same way as in the true Vertebrata. 



Some of the Tunicates (Compound Ascidian s) form 

 colonies, throwing off thread-like outgrowths, from which new 

 individuals develop, in other respects independent of each other. 

 In other colonial forms, Ascidioe compositm, there is a common test: 

 in this case the colonies are firmly attached to some foreign body, and 

 form soft spongy masses, in which the individuals are embedded, 

 ■often on a stellate plan. The members of each group have usually 

 a common cloacal opening, but each has its own mouth. The pelagic, 

 free-swimming, phosphorescent Pyrosoma, also belongs here. 

 This colony has the form of a thick-walled tube, open at one end, 

 closed at the other, walls being formed by the tiny animals which are 

 arranged close together, their mouths opening at the surface of the 



* The mantle is to be regarded as a much thickened cuticle. It is interesting 

 to note that it contains scattered cells, which are amoeboid, and have migrated 

 from the mesoblast through the epidermis into the mantle. 



