382 GILMAN A. DREW. 



These colours are imparted to the genital organs. As their 

 products begin to mature, the genital organs become very 

 extensive and crowd between and around other organs, until 

 all available space is filled. 



The genital ducts of the adult, as in the young, connect 

 with the outer ends of the kidneys, and with them open into 

 the mantle chamber. 



Summary. 



The young embryos of Nucula delphinodonta and 

 Yoldia limatula resemble each other in most respects. 

 They differ considerably in appearance, because of the dif- 

 ference in the size and distribution of the surface cilia. In 

 the case of Yoldia the apical cilia are long and bunched to- 

 gether, and the cilia on the three intermediate rows of test- 

 cells are collected into bands (Text-fig. F). In Nucula 

 delphinodonta all of the cilia on the surface of the embryo 

 are short and evenly scattered (Text-fig. E). The embryos 

 of Yoldia swim freely in the water, and have to depend on 

 their own activities for safety. The embryos of Nucula 

 delphinodonta develop in a protecting brood-sac (fig*. 1). 

 It is to the advantage of these embryos to remain in the 

 brood-sac, so active locomotion would not only be of no 

 value, but it would be a positive danger. The possession of 

 a test that is not functional as an organ of locomotion pro- 

 bably indicates that the embryos of the ancestors of Nucula 

 delphinodonta were free-swimming. They then probably 

 corresponded closely in appearance to the embryos of Yoldia 

 limatula and Nucula proxima, both of which have the 

 apical tuft and the bands of cilia. 



The presence of a separate anal opening in the test, an ex- 

 tensive apical plate, and the formation of the cerebral ganglia 

 without invaginations (fig. 24), are points in which Nucula 

 delphinodonta differs from Yoldia. Nucula delphino- 

 donta sheds its test when the foot is very immature. 



The development of many of the organs of Yoldia has not 



