505 



point the canals swell out and end. Beyond the swollen end of each 

 canal, a thin strand of fibrous tissue is continued on for some di- 

 stance. Sometimes one or more little closed pouches, op, are found 

 lying in this strand of tissue. That the canal and pouches are closed 

 has been repeatedly verified by sections. 



The strand of tissue that continues past the otocystic canal, to- 

 gether with the pouches that it sometimes contains, seems to indicate 

 that the otocystic canals of Y. limatula have undergone, and are per- 

 haps still undergoing degeneration 1 ). 



Each otocyst of Y. limatula contains a single large otolith, ol, 

 that plainly shows concentric structure. The otocysts of both species 

 of Nucula contain numerous irregular particles. 



Circulatory System. 



The hearts of Y. limatula and N. proxima are perforated by the 

 intestines. The heart of N. delphinodonta lies above the intestine. 

 It is interesting to note that with two species of the same genus, the 

 heart in one case is perforated by the intestine and, in the other 

 case, it is not perforated by the intestine. Stempell (10) reports 

 that the heart of Malletia chilensis lies below the intestine. We ac- 

 cordingly have in this group the three possible conditions. 



All three forms are provided with anterior and posterior aortae, 

 which leave the heart from opposite ends. 



Embryology. 



Just before the females of N. delphinodonta lay their eggs, each 

 builds for them a thin walled egg case, Fig. 8, which is attached to 

 the posterior portion of the shell, and opens into the mantle-chamber. 

 This case is composed of a mucous-like material, with foreign bodies, 

 sand, diatoms, fragments of eel grass, &c. adhering to and imbedded 

 in it. 



Although the act of making the case has never been observed, 

 examination of specimens seems to indicate that the mucous-like material 

 is secreted by the epithelium of the mantle or by the epibranchial 

 gland. This material is probably passed posteriorly by the action 

 of cilia and, very likely the respiratory currents of water swell the 

 material into a sort of bubble that remains attached to the posterior 



1) Considering the differences in structure of members of this group, 

 it may be possible that specimens of Y. limatula collected in other parts 

 of the world, have the otocystic canals opening at the surface of the foot. m 



