STUDIES IN THE EMBRYOLOGY OF ECHINODERMS. 417 
the coelom in this group I must refer to the well-known 
accounts of Metschnikoff (18) and Selenka (25). In a well- 
developed Auricularia, we find a pair of enterocoels beside the 
stomach, and a single vesicle on the left side at the level of 
the oesophagus ; this vesicle, which opens to the exterior by 
the water-pore, is usually looked upon as the hydrocoel, but I 
shall endeavour to show that it contains also the rudiment of 
an anterior enterocoel. 
Fig. 22 gives a lateral view of this vesicle in an abnormal 
specimen which first attracted my attention to the subject : it 
will be seen that between the straight tube leading from the 
pore and the thick-walled inner portion (which subsequently 
becomes lobed, and is undoubtedly the hydrocoel), there inter- 
venes a thin-walled section, which extends but slightly behind 
the pore, but is considerably elongated towards the anterior 
end of the larva. 
In fig. 21 we have a dorsal view of the same part of another 
larva, showing that the thin-walled cavity has no great lateral 
extension. It is evident that this cavity has precisely the 
position and relations of the anterior enterocoel of an Echinid 
Pluteus, or of the small Bipinnaria above described; it is, 
therefore, important for us to see how far it is represented in 
normal larvae. My attention was not called to this point till 
rather late in the season, when Auriculariae were becoming 
scarce ; but from time to time I managed to obtain a fair 
number of larvae, and there was not one in which I was not 
able to recognise some vestige of this anterior enterocoel, 
though before noticing the abnormal one (fig. 22) I had never 
detected a trace of such a structure. The cavity is extremely 
variable in size, but fig. 23 represents what appears to he a 
fairly typical development, and is useful in illustrating the 
extreme difficulty of observation. Owing to the form of the 
larva, the only positions in which it is possible to get a steady 
and prolonged observation, give us a directly dorsal or directly 
ventral view ; but in a dorsal view (fig. 23) the thin-walled 
anterior enterocoel lies directly over the thick- and refringent- 
walled hydrocoel, and is consequently extremely difficult to see, 
