STUDIES IN THE EMBRYOLOGY OF EOHINODERMS. 
423 
ciliated epithelium. It runs directly dorsalwards, and after an 
extremely short course* opens into the comparatively thin- 
walled anterior enterocoel almost immediately below the pore. 
From the fact that it is almost impossible to get anything but 
a directly dorsal or directly ventral view of a living Ophiurid 
Pluteus, the relations of the parts just described are not 
always easy to make out ; and owing to the extreme minute- 
ness of the cavities concerned* sections are even less satis- 
factory than the living objects. Nevertheless, with favourable 
living specimens (Pluteus paradox us is one of the most con- 
sistently satisfactory forms* but there is much individual 
variation) placed in an extremely small quantity of water, and 
examined with a high power (Zeiss* Obj. E or F), it is not 
usually a very difficult matter to see the cilia working in the 
water-pore at the surface, and in the up-turned mouth of the 
water-tube at a deeper level. 
In fig. 3 I have attempted to give some idea of a dorsal 
view of this region of the body, but it is impossible in such a 
figure to convey a correct notion of the differences of level, 
and the arrangements of the parts will be better understood 
from the diagram (fig. 2). 
Shortly before the final metamorphosis of the whole larva 
into the pentagonal form, the hydroccel grows round the 
oesophagus into the form of a ring. This has been already 
described by Metschnikoff, but I have not found it easy from 
his description to tell in which direction this growth takes 
place, and as others may have shared my difficulty, I may be 
excused for adding some details to his account. 
Fig. 4 gives nearly all that is necessary ; the fourth and 
fifth lobes retain their places, while the three anterior ones 
grow across to the right side on the dorsal side of the 
oesophagus. On reaching the right side, the first lobe passes 
underneath (ventral to) the anterior enterocoel and the oeso- 
phagus, and so nearly joins the fifth, which at the same time 
bends slightly in under the oesophagus as if to meet it. 
The further history of these parts cannot be described with- 
out entering into a detailed account of the metamorphosis, but 
