STUDIES IN THE EMBRYOLOGY OF ECHINODERMS. 409 
Studies in the Embryology of the Echinoderms. 
By 
II. Bury, B. A., F.I*.S. 
Fellow of Trinity College, Cambridge. 
With Plates XXXVII, XXXVIII, and XXXIX. 
In the following pages I propose to give a comparative 
account of the structure and development of certain organs in 
young Echinoderm larvae. I am not here concerned with the 
earliest stages (gastrula, &c.), nor with those later ones in 
which the pentamerous symmetry of the adult is already 
assumed, but only with that bilaterally symmetrical stage 
which is more or less clearly represented in all Echinoderm 
larvae, and to which Semon (28) has given the convenient name 
“ Dipleurula.” 
At this stage but few organs are developed, and one of them 
— the alimentary canal — is too similar in structure throughout 
the group to need any comparative description. I have there- 
fore confined myself to the following points : 
I. The primary divisions of the coelom, starting 
from a stage in which at least two enterocoel pouches 
are already present. 
II. The Hydrocoel: its development and connections. 
III. The Skeleton, so far as it is developed in the dip- 
leurula stage. 
I. Primary Divisions of the Ccelom. 
Up till quite lately only three main divisions of the coelom 
were usually recognised in Echinoderm larvae — the right and 
