676 THE INVERTEBRATA 



Meanwhile the gill clefts increase in number by the breaking through 

 from the pharynx of new clefts and the subdivision of existing clefts, 

 in the course of which they pass through a U -stage with tongue bars. 

 (The basket work is ultimately established by the formation across 

 each gill cleft of longitudinal bars which divide it into stigmata.) From 

 the pharynx leads the rest of the alimentary canal, which early shows 

 rudiments of oesophagus, stomach, and intestine, the latter curving 

 dorsally and eventually opening into the left half of the atrium. 



d.n.c. 



trTcga. 

 int. I fi. 



A* 





atop. 



Fig. 472. Diagrams of the metamorphosis of an ascidian larva. A, The larva 

 at the time of fixation. B, Midway in the metamorphosis. C, The meta- 

 morphosis completed, ad.ga. adult ganglion; at. right rudiment of the 

 atrium; at.op. atrial opening; ce.ves. cerebral vesicle; ci.f. ciliary funnel ; 

 d.n.c. dorsal nerve cord; e. eye; epic, epicardium; est. endostyle; fix. fixation 

 papillae; ga. ganglion; g.s. gill slits; h. heart; int. intestine; M. mouth; nch. 

 notochord; st. stomach; stat. statolith; trk.ga. trunk ganglion. 



The dorsal nerve tube of the tail is in the trunk enlarged to form 

 the brain. The hinder part of this is thick-walled and is known as the 

 trunk ganglion (it does not become the "ganglion" of the adult). The 

 anterior part is larger than the trunk ganglion and for the most part 

 thin-walled, and is known as the cerebral vesicle. Dorsally on the 

 right it is differentiated to form the eye, a cup whose cavity is directed 

 inwards, filled with pigment. On the floor a stalk projecting into the 

 vesicle carries a concretion, the statolith, probably a sense organ for 



