218 Mr. G. Lefevre on 
through the wall of the latter. At about the time when the 
fold of the right peribranchial sac first appears the gut is 
formed as a blind diverticulum at the posterior end of the 
branchial sac. At the point of origin it turns sharply towards 
the left, and, growing rapidly upwards and forwards against 
the outer surface of the left peribranchial sac, it finally reaches 
the mid-dorsal line, where it opens eventually into the median 
portion of the peribranchial cavity. 
In fig. 2 is seen a groove (HND.) on the inner surface of the 
endodermal vesicle just below the pericardium. This is the 
rudiment of the endostyle, but when it first appears its position 
is much higher up than it is in this figure ; during the rota- 
tion the relative position of pericardium and endostyle is not 
changed. 
By a study of a most complete series of stages I have been 
led to conclusions in regard to the origin of the common rudi- 
ment of the dorsal tube and nerve-ganglion in Perophora 
