THE LIFE-HISTORY OF NUCULA DELPHINODONTA. 825 



acquires an anus (fig. 24). The anus is not directly applied 

 to the pore that opens between the test cells, but it opens 

 into a cavity that is continuous laterally with that portion of 

 the embryo that, as the inantle continues to grow, becomes 

 the mantle chamber. This communication will be described 

 in a later stage. 



The embryo flattens laterally until its thickness equals 

 about two thirds of its dorso-ventral width, and the dorsal 

 space becomes considerably enlarged (fig. 25). Near the 

 anterior end of this space the anterior adductor muscle [aa.) 

 makes its appearance. At first it consists of a very few 

 fibres, and is not conspicuous. The anterior eularged 

 portion of the gut takes on the distinctive characters of the 

 stomach [sto.), and the liver grows out as paired right and 

 left pouches (/.). The anterior end of the stomach is carried 

 dorsally, and a more or less distinct bend is made where it 

 joins the intestine. 



The relationship of the various cavities in the embryo to 

 each other, and of the anal pore in the test to the mantle 

 chamber, can be best understood by comparing the sagittal, 

 horizontal, and transverse sections of embryos, represented 

 on Plate 22, with the reconstruction of an embryo at the same 

 stages of development (Plate 21, fig. 25). The position of the 

 horizontal and transverse sections are indicated on fig. 25 by 

 numbers that correspond to the numbers of the figures. 



The dorsal cavity is separated from the ventral cavity by 

 the gut (fig. 28). In some sections the two cavities communi- 

 cate around the sides of the gut. This may be due to 

 shrinkage, but it seems more likely that the two portions are 

 parts of a single cavity. It is just possible that the cleavage 

 cavity never entirely disappears, and that this cavity can be 

 traced back to the blastocoele, but I am of the opinion that it 

 is a later formation, and represents a schizoccele. Its fate is 

 of interest, and will be referred to in later stages. 



The lobes of the mantle are now well formed, a distinct 

 shell-cuticle has been secreted, and some lime salts have been 

 deposited. The stomodaeum for most of its length is joined. 



