20 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



Sometimes one or more little closed pouches are found, lying in 

 the strand of tissue that is continued on past the swollen portion of 

 the canal. Two such pouches are- shown in Fig. 22, which represents 

 the distal swollen portion of a canal and part of the continuing strand 

 of tissue. These pouches, when found, show the same structure that 

 is shown by the canals, and like them contain numerous, very long cilia. 



It seems to me that these walls and pouches show conclusively 

 that in each of the several cases the whole canal was removed 

 and, that in this species, the canals do not reach the surface of the 

 foot.* 



In both Nucula proxima and Nucula delphinodonta there is a 

 strand of tissue passing from each otocyst to the surface of the foot. 

 This corresponds in position to the canal described by Pelseneer 11 for 

 Nucula nucleus, and is probably an open canal, but I have not had 

 time to demonstrate this point. 



In Yoldia limatula, the strand of tissue, with the series of pouches 

 which it sometimes incloses, seems to indicate that these canals have 

 undergone degeneration. 



The otocystic nerves lie alongside the canals and seem to send 

 fibres to their walls. In fact it is difficult to demonstrate that the 

 nerves reach tile otocysts other than through the walls of the canals. 



OSPHRADIUM. 



On the inner side of each suspensory membrane of the gills, lying 

 just beneath the visceral ganglion of the corresponding side, is a rather 

 large patch of modified epithelium, which probably represents an 

 osphradium. 



This epithelium is supplied by a rather large nerve that leaves 

 the corresponding visceral ganglion near its anterior end. 



CIRCULATORY SYSTEM. 



The heart, Fig. 4, is situated just posterior to the beaks of the 

 shell, and very near the dorsal margin. The cavity of each auricle is 

 connected with the blood-spaces of the corresponding gill, through a 



* Perhaps individuals living in widely separated localities may differ in this 

 regard. 



