12 MR. E. RAY LANKESTER ON THE 
A study of the later phases of Pistdiwm pusillum would no doubt throw some light 
on the origin of these structures as well as on the origin of the labial tentacles, the 
nature of which, especially in relation to the branchiz, requires investigation. 
Explanation of the lettering of the figures in Plates 1, 2, 3, 4. 
a. Anus. 
al. Central portion of the alimentary tract. 
Ad. Anterior adductor muscle. 
B. Rudiments of Bosanus’s organs. 
br. Branchial buds. 
ci. Cilia. 
ci. Cilia of the mesoblastic cavity. 
chy. Lumen of the hypoblastic invagination. 
ep. Epiblast. 
f. Foot. 
hy. Hypoblast formed by invagination. 
tr. Lumen of the rectum. 
me. Mesoblastic cells. 
mn. Mantle-flap or border. 
o. Mouth. 
p. Cells apparently in the act of budding off from the hypoblast to form meso- 
blastic elements. 
pd. One of these cells dividing into four. 
ph. Pharynx. 
rp. Rectal peduncle of the hypoblast. 
rc. Czecal termination of the alimentary canal. 
sh. Shell-groove or shell-gland. 
v. Problematic vesicle. 
a. Large cells doubtful as to being epiblastic or mesoblastic. 
y. Large cells persisting until late development in the epipharyngeal region. 
