124 



THE VOYAGE OF H.M.S. CHALLENGER. 



tail-like part of the body, where it would form an axis around which the muscles were 

 placed. This would account for the change in the general shape, in the disposition of 

 the alimentary canal, and in the relations of the notochord. The other changes which 

 probably took place in the primitive Tunicata were the enlargement of the anterior 

 part of the nervous system to form a vesicle in which sense organs (optic and auditory) 

 were developed ; the degeneration and abortion of the nephridia, with the possible 

 exception of those underlying the nervous system in the prseoral lobe which later on 

 may have become modified to form the subneural gland of the Ascidian, the homologue 

 of the hypophysis cerebri of vertebrates ; and the enlargement of the anterior portion 

 of the alimentary canal to form a respiratory tract — the primitive Ijranchial sac. 



Most of the AppendiculariidfB of the present day (see Fig. 14) represent such an 

 ancestral form with comparatively little change. They have the Ijranchial sac or 



Fig. 14.— Diagram of Ajipcndicularia from the right side. (From the Encyclopajclia Eritannica, 9th ed. ) 



a. anus ; app. tail ; at. one of the atria! apertures ; In: branchial aperture ; br.s. branchial sac ; d.t. dorsal tubercle ; 

 end. endostyle; h. heart; ■('. intestine; m. muscle band of tail; n. nerve cord in body; re.' nerve cord in tail; 

 n.ff. large anterior (cerebral) ganglion ; n.ff.' caudal ganglion ; n.ff.' enlargement of nerve cord in tail ; 

 cc. tesophagus ; ot. otocyst ; ov. ovary ; p.p. peripharyngeal band ; sg. one of the stigmata ; s.o, oral sense organs ; 

 St. stomach ; tcs. testis ; ii. urochord ; it.' its cut end. 



pharynx opening to the exterior by a single pair of stigmata or gill slits which 

 correspond to the primary or first formed stigmata of the young Ascidia, and may 

 represent the ancestral condition. In some few of the Appendiculariidse, however, 

 considerable degeneration has taken place (e.g. KotvcdevsJcia) ; and in one form 

 described as new, but not named, by Dr. Moss in 1871,' the branchial sac has a 

 number of stigmata, and is in a condition which recalls the arrangement found in the 

 Doliolidse. This species, which may be called Ajjpendicidaria mossi, is perhaps the 

 nearest form known to the ancestral Tunicates from which the two ereat lines of 

 degeneration diverged (see table, p. 120), the one leading to the Doliolidc'e and the 

 Salpidae, and the other to the Simple and the Compound Ascidians. 



' Trans. Linn. Soc. Land., vol. xxvii. p. 299. 



