xvii 



PEOTOCHOEDATA 



639 



acquires a new opening into the vesicle at the front end This 

 new opening becomes the sub-neural gland, or hypophysial tube; 

 the other opening disappears- No protostigmata are formed; the 

 definitive stigmata arise as small perforations of the pharyngeal wall. 

 The intestine arises as an outgrowth from the hinder 'wall of the 



Pig. 464. — Stages in the development of the bud of Distaplia magnilarva. (After Hjort.) 



A, lateral view of young bud in which the two atrial rudiments are still separated. B, dorsal view 

 of young bud in which the two atrial rudiments are still separated. C, dorsal view of older bud in which 

 the atrial rudiments have united in the mid-dorsal line, at.l, left atrial rudiment ; (d.r^ right atrial 

 rudiment; ep, epicardial tubes; int^ rudiment of intestine; n.t, nerve tube — rudiment of adult ganglion ;' 

 ov, ovary ; jsTt, pharynx. 



pharynx ; the pericardium as a median evagination of the ventral 

 wall of the same organ. The muscles and the genital organs arise 

 from the mesodermal cells (Fig. 464). 



In Pyrosoma, Salpa, and Doliolum, the stolon arises in just 

 the same manner as in Clavelina. In Pyrosoma, as we have already 



