140 Messrs. W. K. Brooks and G. Lefevre on 



not appear until a very late stage, and then merely as prolon- 

 gations backward of the peribranchial cavity, after the latter 

 has been entirely severed from the inner vesicle. This would 

 therefore be a still more modified condition than that which 

 occurs in Boiryllus. 



Pizon makes the statement that in the buds of Perophora 

 Lister i he has found that epicardial tubes arise as two diver- 

 ticula from the posterior end of the branchial sac, in the same 

 manner as has been described for Amaroncium, Clavelina, 

 &c. Such an account is not in the slightest accord with our 

 observations, and if epicardial tubes arise in this manner in 

 the buds of the European species, they certainly do not in 

 Perophora viridis. 



The Branchial Sac. 



That portion of the primitive inner vesicle which is left 

 after the separation of the peribranchial cavity becomes the 

 branchial sac or pharynx. The thickened right wall of the 

 vesicle, as already stated, is shifted or pushed down until it 

 comes to lie on the ventral side, wliere it forms the floor of 

 the future pharynx. 



Very soon after the vesicle has begun to change its position 

 a slight longitudinal groove is found on the inner surface of 

 the vesicle high up on the right side. It runs through the 

 middle of the thickened area — that is to say, above and below 

 it there are equal portions of the thick wall, which will lie on 

 its right and left, when by the displacement of the vesicle the 

 groove is brought down to the mid- ventral line. By the 

 deepening and widening of this groove and by the differentia- 

 tion of the cells bordering upon it the definitive endostyle is 

 formed. 



After the appearance of the gill- slits, each of which arises 

 as an independent formation, the pharynx is put into commu- 

 nication with the peribranchial cavity. 



The ectoderm of the bud becomes greatly thickened at a 

 point opposite the extreme anterior end of the branchial sac, 

 and invaginates until it touches the wall of the latter. A 

 complete fusion then takes place, an opening breaks through, 

 and the branchial orifice is established. 



The cloacal orifice is formed in exactly the same way, by 

 the union of an ectodermal invagination with the wall of the 

 cloaca at the anterior end of the latter. 



The Digestive Tract, 

 Some time before the displacement of the inner vesicle is 



