620 INVEETEBEATA CHAP. 



surfaces of the trunk a pair of invaginations make their appearance. 

 These are the atrial invaginations ; they meet and fuse each witli 

 two evaginations of the pharynx, and thus are formed the first two 

 pairs of gill-slits, which, for reasons to be given later, we designate 

 as gill-slit No. 1 and gill-slit No. 4. 



From the hinder wall of the pharynx there grows out a tube- 

 like diverticulum, the rudiment of the adult intestine. The end 

 of this curves up the left side of the body, and in the larvae of some 

 species even comes into contact with the ectoderm, not far from the 

 mid-dorsal line; but no anus is formed till the -free -swimming life is 



ni 



FIG. 450. Longitudinal sagittal section of tadpole of Cynthia partita just before it 

 escapes from the egg-membrane. (After Conklin.) 



b.v, brain-vesicle ; eft, notochord ; nis, mesodermal band (seen by slight obliquity of the posterior end 

 of the section) ; n.t, nerve tube ; ph, pharynx ; v.end, ventral cord of endoderm cells in the tail. 



over. Along the ventral wall of the pharynx there is differentiated 

 a broad strip of columnar cells ; this is the endostyle. 



Finally, the wall of the brain-vesicle undergoes modification. On 

 the right side a hemispherical outgrowth takes place, and the cavity 

 of this cup becomes filled up with a secretion which becomes 

 pigmented. This is the cerebral eye of the Ascidian tadpole, and 

 corresponds, without doubt, to one of the two optic vesicles of the 

 brain of a higher Vertebrate. Close behind the eye rudiment there 

 is another pigrnented area of epithelium which is the rudiment of 

 the ear or balancing organ. It is soon separated from the eye and 

 forced round to the floor of the brain-vesicle by an expansion of the 

 epithelium between it and the eye, which thus gives rise to the thin- 

 walled epithelium forming the greater part of the wall of the brain- 

 vesicle. The ear eventually forms a median prominence, shaped like 



