o20 



TUNICATA. 



to derive the condition of the gill in the Salpidae from that in the 

 Doliolidae, and not compare it, as a primitive structure, directly with 

 that of the Ajipendicularia, as was done by van Beneden and Julin 

 (see p. 566). The curious transformations of which the organisation 

 of the Tunicates is capable is shown by the extraordinary Octacnemidak 

 which live at the bottom of the sea at great depths ; these star-like 

 creatures, which resemble Lucernaria in form, must probably be 

 regarded as modified Salpidae (Herdman, No. 29, Pt. III.). 



Since Kowalevsky laid the foundations for our knowledge of the 

 ontogeny of the Ascidians and of Amphioxus there has been no room 

 for doubting that the two branches of the Tunicate stock and the 

 Cephalochorda are intimately related. Later researches have entirely 

 confirmed this conviction. The agreement found in the most 

 important points of organisation and of development can only rest 

 <>n true homology. Among these are : the presence of a dorsal 

 neural tube which, in early stages, opens externally through the 

 neuropore; the possession of a chorda dorsalis extending between 

 this and the alimentary canal ; the transformation of the anterior part 

 of the alimentary canal into a respiratory region perforated by lateral 

 gill-slits, the ventral side of which region is occupied by the hypo- 

 branchial groove (endostyle) ; and, finally, the transformation of 'the 

 posterior part of the body into a locomotors organ provided with an 

 unpaired marginal fin, while the intestinal rudiment at this part 

 degenerates. The first stages of development in these two forms 

 are also strikingly alike. Only recently van Beneden and Julin 

 (p. 349), who, however, are not supported by Davidoff, have made 

 statements as to the development of the mesoderm and the chorda 

 dorsalis winch would render the agreement between these two groups 

 almost complete. We are, therefore, justified in regarding the 

 Tunicata and the Cephalochorda which lead on to the Vertebrates, 

 as members of a large common group, the Chordata, and to derive 

 them from a common racial form (the Protochordata). We must 

 imagine this racial form as a pelagic, segmented animal, provided 

 with gill-slits and a chorda. The Tunicates, as compared with such 

 a form, are to some extent degenerate, this being due, on the whole, 

 to their attached manner of life, while, in another direction, their 

 organisation is more highly developed. This is the case, for instance, 

 in the pharyngeal region which has undergone considerable enlarge- 

 ment and specialisation. 



Among the indications of degeneration found in the body of the 

 Tunicates we should first mention the loss of the coelom and the 



