320 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
curves round towards the posterior end of the larva. 
These are soon separated from the archenteron by a 
constriction (fig. 40), and then form a single horse-shoe 
shaped hydro-enteroccelic vesicle (hy.ent.). Concurrently 
with the formation of the vesicle, or immediately following 
it, the two limbs of the horse-shoe become separated by 
further constriction, and lie on the sides of the archenteron. 
The two vesicles again divide into anterior and posterior 
portions; and the anterior portion of that on the left 
ultimately opens to the exterior through a water pore, and 
gives origin to the water-vascular system of the adult. 
The remaining vesicles are enteroccelic, and ultimately 
form the lining of the body cavity. After the separation 
of the hydro-enteroccel vesicle, the blind end of the 
archenteron bends towards the ventral surface, in which, 
at the same time, a small invagination of the ectoderm 
takes place. The two eventually meet, and by the disap- 
pearance of their walls at the point of union, become 
continuous. The ventral invagination becomes the mouth, 
and the blastopore the anus of the piuteus. Three pairs 
of arms, in addition to the posterior ventral pair already 
mentioned, make their appearance and gradually increase 
in length as growth of the pluteus as a whole proceeds. 
The body becomes shorter, and its posterior end well 
rounded (Pl. V., fig. 41). 
The metamorphosis of the pluteus into the young 
Echinoid has not yet been fully described in our species. 
A flask-shaped invagination of the ectoderm of the left 
side grows inwards towards the hydroccel vesicle, to the 
exterior of which it is applied as the ‘‘Hchinoid disc.” 
Its floor thickens and becomes discoid, while from the 
hydroccel five primary tentacles grow out, pushing before 
them the Echinoid disc, which is said to form the epithe- 
lium, and possibly the radial nerves, of the oral half of the 
