14 
ON A NEW SPECIES OF ENTEROPNEUSTA, 
adjoining the ventral wall of the neck of the notochord and the 
curved side above the divaricated legs of the Y-shaped " keel." 
At this point there passes in the " chondroid tissue " between the 
" body " and "keel" a vessel connecting the two efferent proboscis 
vessels (fig. 10, cv.) Immediately behind this vessel the " body " 
becomes triangular in outline and its apex gradually approaches 
and finally fuses with the median portion of the " keel " between 
its divaricated legs which now give rise to two lateral "wings." 
Anterior to and at the point of fusion the body is much stouter 
than the keel, the "wings" of the latter only projecting a very 
little beyond the lateral surfaces of the body, but posterior to this 
the body gradually decreases in breadth and also in height, while 
the keel thickens greatly, forming in transverse sections much the 
lugger half of the whole structure. The "wings" of the "keel " 
at the same time reach a greater lateral extension and form two 
distinct lateral projections below the middle region of the body 
when the entire skeleton is viewed from above (fig. 13). Poste- 
riorly the wings become gradually smaller and finally disappear, 
while the body becomes reduced to a narrow somewhat convex 
plate separated from the keel by two small vertical half moon- 
shaped masses with their convex faces touching each other. 
These are the first indications of the two " legs," and for them 
Spengel adopts Bateson's term "nuclei." After the first appear- 
ance of the " nuclei " the keel gradually becomes reduced in size, 
the "nuclei" at the same time becoming larger and more distinct. 
The place where the "nuclei'' first touch in the middle line corre- 
sponds, as Spengel has shown, to the most anterior point the 
opening of the notochordal lumen into the mouth cavity has 
occupied. In transverse section the proboscis skeleton has, just 
after the proboscis neck has fused with the collar, a triangular 
shape, but gradually as the "keel" is reduced in size and the 
" nuclei " become larger and more distinct, the shape becomes 
quadrangular and the skeleton then consists of a dorsal plate, 
representing a continuation of the " body " and derived from the 
notochord, a middle portion formed by the two semilunar 
"nuclei" derived from the throat epithelium, and a ventral plate 
