598 THE INVERTEBRATA 



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 balance. Presently the vesicle acquires an opening into the 

 dorsal side of the pharynx, near the mouth. The pericardium arises 

 towards the end of larval life as an outgrowth from the ventral side 



ir}:.(ja.^ 



at. op. '^-i^^^^ C 



Fig. 447. 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 ; yix. fixation 

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

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



of the pharynx. It does not form the heart until metamorphosis. 

 The front end of the body is a prominent chin, and bears three fixation 

 papillae of glandular cells. Except for the tips of these papillae, the 

 animal is entirely covered with test, which even closes the mouth, so 

 that feeding is impossible. After swimming for a short time, the larva 

 fixes itself to some solid object by the papillae, and proceeds to under- 

 go a metamorphosis (Fig. 447 B, C), by which it assumes the adult 

 form. The tail is devoured from within by phagocytes. By growth in 

 the region between the chin and the mouth, the latter and the atrial 



