DEVELOPMENT OF ASCIDIAN 



507 



region, and pharyngeal gill-slits, swims for hours by its tail, and then 

 fixes, head downwards, by three glandular papillae. 



Degeneration begins. Two main processes go on concurrently : — 

 (i) the disappearance of the tail, and with it the notochord and most 

 of the nervous system ; (2) the growth of test, pharynx, and gonads. 

 The tail is consumed by phagox:ytes. The peribranchial chamber 

 becomes greatly enlarged, opening at the atrial aperture. The pharynx 

 becomes large, and the gill-slits increase in number. Exceptional 

 upward growth of the originally short region between fixing papilla? 

 and mouth results in the adult relation of parts. The whole meta- 

 morphosis is one of the most signal instances of degeneration. 



en. 



Fig. 288. — Embryo of Clavelina. — Modified after Seehger. 



f.p., Fixing papilla ; e.f., ectodermic fold ; e.g., ciliated groove ; 

 en., endostyle ; s.o., cerebral vesicle with sense organs ; g.s., 

 gill-slits ; n., nerve-cord beginning to degenerate ; c/?., noto- 

 chord ; g., gut curving upwards towards atrial opening. The 

 atrial invagination is marked by a dotted line ; the mouth and 

 atrial opening are indicated by arrows. 



General Notes on Tunicata 



The description of Ascidia given above is, in its general 

 outlines, applicable to all the simple Ascidians, which are 

 abundantly represented on British coasts. As contrasted 

 with this type, we have in other members of the class most 

 remarkable diversity in structure, habit, and life-history. 



The simple Ascidians are usually sedentary, growing 

 fixed to stones, shells, or weed, and are widely distributed, 

 occurring on or near the coasts of all seas. With the 

 exception of the so-called social Ascidians {e.g. Clavelina), 

 they do not reproduce by budding, but are often gregarious, 

 great masses being found together. 



To the compound Ascidians {e.g. Botryllus) the simple 

 forms are linked by Clavelina, where each individual is 



