WITH NOTES ON THE WEST INDIAN SPECIES. 



251 



tissue, while the left canal opens widely into a median end-vesicle terminating in 

 an ill-defined pore luhich may open into the base of the medullary tube someiuliat behind 

 the anterior neuropore^ (PI. XXIX. Fig. 17 a — c). Behind the proboscis-pore, the ventral 

 angles of the end-vesicle are continued for a short distance below the medullary tube 

 as a pair of coecal pockets which may be separate or united. The association, here 

 described, of the proboscis-pore and medullary tube, suggests morphological relationships 

 of great significance and complexity of which I had previously no idea. The pore does 

 not form a gaping orifice but is narrowed or even subdivided by a reduplication of the 

 wall of the end-vesicle. The meaning of this reduplication will be apparent when we 

 come to consider the West Indian species. In a younger specimen I find a simple 

 undivided median end-vesicle opening to the exterior by a median pore in front of 

 the anterior neuropore. The reduplication of the end-vesicle 'would therefore appear to 

 be secondary in an ontogenetic sense — a fact of some interest. 



Stomochord. 



The coecal dilatation of the stomochord is remarkable for the feeble development 

 of the lateral pockets, and, connected therewith, its relatively small transverse 

 expansion. The expansion in the dorso-ventral direction is approximately normal ; in 

 the transverse direction it is less than usual. There are both dorso-lateral and ventro- 

 lateral subdivisions of the stomochordal lumen (PL XXIX. Fig. 17 a). 



Tracing the stomochord in section from before backwards, its lumen is seen to 

 be in a vestigial condition (reduced to zero or interrupted and broken) until near 

 the posterior end of the coecal dilatation. Here the lumen widens out and the cells 

 composing its dorsal wall are densely ciliated. This is an unexpected result, and the 

 reason for the occurrence of cilia in this position is not very apparent, since the 

 continuity of the stomochord is interrupted some distance in front of its buccal orifice. 



Behind the coecal region the stomochord becomes abruptly reduced in bulk and 

 continues to decrease in size until it reaches a point where it positively splits up 

 into three minute portions separated from one another by processes from the dorsal 

 edge of the nuchal skeleton (PI. XXIX. Fig. 18). Finally these fragments of the 

 stomochord unite with, and are absorbed into, the ventral wall of the wide terminal 

 division of the stomochord, the dorsal wall of which is again finely ciliated. A similar, 

 but more extensive fragmentation of the stomochord in the nuchal region, through its 

 being traversed by bridges of skeletal substance is described by Spengel in Bal. kiipfferi 

 (Spengel, Hon., Taf XV. Figs. 24—26). 



Nuchal Skeleton. 



The dorsal edges of the cupule of the nuchal skeleton are produced forwards as 

 two thickened skeletal bands lying above the ventro-lateral pockets of the coecal dilatation 

 of the stomochord (PI. XXIX. Fig. 17 a). The body of the skeleton is produced dorsally 

 into a high crest which is instrumental in effecting the fragmentation of the stomochord 



' Similar observations are recorded by Spengel in Bal. canadensis and Gl. talahoti. [Spengel, Mon. Taf. 17, 

 Fig. 13, and Taf. 19, Fig. 6, page 607.] 



