128 HUGH WATSON. 



and more calcareous, especially in full-grown specimens, though 

 it is usually very brittle. Indeed in some examples of A. 

 gibbonsi its thickness is sufficient to suggest that the chief 

 function of the shell in this species and its allies may be to 

 enable the animal to get rid of any excess of calcium 

 cai'bonate in its blood. It is difficult to see what other 

 function it could fulfil ; and if it were to be regarded as a 

 purely vestigial structure with no function at all, one would 

 have expected it to have become much smaller as well as 

 flatter in this species than in the more primitive forms such 

 as A. purcelli. Yet, while the shape has degenerated, the 

 relative size has remained the same nnd the thickness has 

 greatly increased. 



The Shell-sac. — The pocket in which the shell lies has no 

 communication with the exterior, but is close to the inner 

 surface of the skin (PI. X, fig. 33). In those species in which 

 the shell has a prominent apex, the floor of the sac is raised 

 towards the hind end to form a papilla fitting into the hollow 

 of the shell. Where the margin of the shell rests upon 

 the floor of the pocket, there is a fold, Avhich separates the 

 central part of the floor underlying the interior of the shell 

 from a nari'ow rim beyond it. 



The shell-sac is lined by a well-defined epithelium. On the 

 roof of the cavity this epithelium is very thin, being formed in 

 Apera dimidia of a compact layer of small cells which are 

 slightly broader than they are high. Underneath the shell 

 the epithelium is thicker in the same species, and composed 

 of larger cells, whose height exceeds their breadth. These 

 larger cells are continued over the edge of the fold mentioned 

 above, the transition from the thicker to the thinner epithe- 

 lium taking place on the outer side of the fold. 



The Mantle-cavity or Lung. — The mantle-cavity occupies 

 the region below the shell and the respiratory opening, and also 

 extends a short distance further forward. It is, nevertheless, 

 mainly behind instead of above the body-cavity or ha3mocoele, 

 from which it is separated by a muscular diaphragm sloping 

 obliquely downwards and backwards. There is, however, a 



