14 ox A NEW SPECIES OF EXTEROPNEUSTA, 



{tdjoining 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 " betw^een 

 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 

 Inofger 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 tlie collar, a triangular 

 shape, but gradually as the "keel" is reduced in size and the 

 " nuclei " ))ecome 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 ]:)y the two semilunar 

 "nuclei" derived from the throat epithelium, and a ventral plate 



