I'HE LIFE-HISTORY OF NUCULA DELPHINODONTA. 32? 



but is not enclosed by the body ectoderm (fig. 28), which, in 

 this region forms the walls of the foot (/.), Near its external 

 opening the stomodaeum has become free, and is more or less 

 closely jointed to the test cells. 



The relation of the anal test pore to the mantle chamber 

 can now be understood. As shown by a sagittal section 

 (fig. 26), this pore opens into a small cavity that receives the 

 anus. This cavity is bounded anteriorly by the posterior 

 wall of the foot, and ventrally either by the stomodseum or 

 by cells covering the dorsal portion of the stomodaeum. 

 Transverse (fig. 27) and horizontal (fig. 31) sections show that 

 this cavity spreads out laterally, and becomes continuous with 

 that portion of the mantle chamber posterior to the foot. At 

 this stage the foot is very imperfectly formed, and contains 

 the cavity that has been referred to as the ventral cavity. 

 The cavity soon disappears, and the ectoderm on the two sides 

 of the foot fuse ventrally, dorsal to the stomodaeum. The foot 

 is still very small, and shows no sign of its future activity. 

 At a corresponding stage the foot of Yoldia is quite well 

 developed (Text-fig. I). This is about the condition of the 

 embryo when the test is thrown away. 



It takes several hours for embryos of this species to cast 

 the test, a process that with Yoldia limatula and Nucula 

 proxima is completed within a very few minutes after it is 

 begun. The test cells in the region of the anal pore break 

 apart, and the whole mass is frequently pushed forward to 

 the region of the apical plate. This stripping forward 

 carries the outer end of the stomodeeum forward to some 

 such position as is shown by fig. 34. The cilia on the test 

 cells remain feebly active for a considerable time. While the 

 test cells, stomoda3um, and apical plate still adhere to the 

 embryo, the stomach and liver pouches are drawn some 

 distance dorsally into the schizocoele (fig. 34, sto. and L). 

 Whether the fibres extending from the stomach to the mantle 

 are important in effecting this movement is not known. Their 

 position is suggestive, but I have no direct evidence that 

 they contract. The position now occupied by the stomach 



