WITH NOTES ON THE WEST INDIAN SPECIES. 231 



Series i. The bulk of the right dorsal canal ends in the chondroid tissue in 

 advance of the termination of the left canal. Before ending it gives off a connecting 

 tube, which proceeds backwards as a solid cord of cells, finally expanding again into 

 the hollow terminal vesicle which is like that of the left side, but smaller. This 

 vesicle opens to the exterior, on the right side, by a small pore in the same trans- 

 verse plane as the much larger pore of the left side (PL XXVIII. Fig. 2). 



On the left side the connecting tube is not solid, the terminal vesicle being in 

 free communication with the left dorsal canal. 



Series ii. The right dorsal canal communicates with a wide terminal vesicle by 

 a narrow connecting tube, whose lumen is occluded. The terminal vesicle opens to the 

 exterior by the right proboscis-pore. On the left side there is no definite connecting 

 tube at all. The left dorsal canal comes to an end in the chondroid tissue, into which 

 it gives off numerous islets of cells, whose presence produces the cartilage-like appear- 

 ance of the chondroid tissue. There is no cord of cells which can be selected from 

 among these cellular islets as being the representative of a connecting tube. Never- 

 theless, there duly occurs, on the left side, a terminal vesicle which is not very much 

 smaller than the corresponding structure on the right side, and opens to the exterior 

 by a sinistral pore in the same transverse plane as the right proboscis-pore. 



Series iii. The condition observed here resembles the preceding, except that 

 the right vesicle is in open communication with the right dorsal canal. The left 

 terminal vesicle is hardly any smaller than the right, but its connecting tube is 

 drawn out into a solid or sub-solid cord. 



Series iv. The right terminal vesicle is in open communication with the right 

 dorsal canal. There is also a terminal vesicle and pore on the left side, but the 

 vesicle is not in open communication with the left dorsal canal, the connecting tube 

 being discontinuous and vestigial. 



Series v. Both end-vesicles communicate with their corresponding dorsal canals 

 of the proboscis-coelom, but the left vesicle is rather larger than the right. 



Series vi. Both end-vesicles end solid internally towards the coelomic canals ; the 

 right vesicle is larger than the left. 



From the above it will be seen that Pt. flava possesses paired terminal vesicles 

 and paired proboscis-pores, but those of one side, either right or left, are larger than 

 those of the other. The larger vesicle, whether it is on the right or on the left side, 

 usually retains more or less of a functional communication with the corresponding 

 dorsal canal. The smaller vesicle, on the contrary, shows a distinct tendency to become 

 disconnected with or emancipated from the coelom of the proboscis. 



Finally, it is to be noted that in this species the terminal vesicles of the dorsal 

 canals do not open to the exterior by a narrow minute pore, but they usually open bodily 

 by a wide orifice, equal in breadth to their own diameter. These pores can be easily 

 seen in situ in uninjured specimens. 



W. III. 33 



