438 
H. BTTEY. 
the plates 1 ), and briefly describe what I find in larvae of the 
seventh and eighth days of development. Fig. 17 is a partly 
diagrammatic view of such a larva from the dorsal (and partly 
left) side, the larval organ being considered anterior, and the 
larval mouth ventral. An oblique mesentery, separating two 
cavities, runs across the posterior part of the stomach, and on 
the left side of it are seen the three dorsal terminals (compare 
fig. 14), while on the right are three basals, the most anterior 
being the madreporite. The cavity on the left is identified 
from Ludwig’s description as the left enterocoel, and this is 
supported by its relations to the terminals ; but the right cavity 
is not mentioned by Ludwig. According to his description it 
too should be part of the left enterocoel, and he recognises no 
mesentery in the position here represented. To confirm this 
view of the whole animal, and to determine the relations of this 
right cavity, I have cut a number of transverse and longitudinal 
sections, and in every case the same result is obtained. This 
cavity is entirely shut off from that of the larval organ, and, 
indeed, from every other cavity; it begins a little behind the 
water-pore, and runs back to the extreme posterior end of the 
larva, being separated from the rest of the enterocoel by a ven- 
tral mesentery, as well as by the dorsal one here represented ; 
it is correctly shown in transverse section by Ludwig (17, 
pi. ii, fig. 37) as a comparatively small cavity exactly opposite 
to the hydrocoel, but there is nothing in his description which 
will enable us to understand how it gets there. I have repre- 
sented it again diagrammatically in fig. 16, in which is shown 
its relation to the dorso-central and basal plates, and the close 
parallelism of the terminals to the mesentery enclosing it. I 
have not yet fully traced its origin or subsequent fate, and 
until I have done so I am unable to determine its true cha- 
racter, or to point out the full extent of the difference between 
Ludwig’s account and mine ; but, bearing in mind the posi- 
1 This statement requires modification : the madreporic plate is called by 
Ludwig the fifth basal, the one immediately behind it being the fourth, and so 
on. I begin with the madreporite and count backwards, as in the case of 
the terminals (fig. 13). 
