40 



Betweon the diiplicature at tlio posterior '.'a^;G of the 

 shell and of the collar, the mantle is very uniform. Its 

 structure io as follows:- Externally, the surface epith- 

 elium is composed of the flattened, non-ciliatod cells, 

 which secrete the calcareous lining of tiie burrow. Inter- 

 nal to the outer epithelium, are the weak m^iscles of the 

 mantle, consisting of the longitudinal layer; a layer in 

 which, the fibres cross obliquely to the longitudinal bi- 

 bres; and internal to these, the circular layer. The in- 

 ternal surface of the mantle (the lining of the mantle 

 cavity) , is lined by cells T;hich in general are columnar 

 and ciliated. Opposite the ends of the gills, the mantle 

 is strongly ciliated and bears numerous mucous gland cells 

 which empty to the surface. This region is indicated di- 

 agrammatically in figs. 29-34 » where on either side v.-en- 

 tral to the ciliated, glandular area the mantle vrall is 

 th.ickened, so as to form a groove opposite the groove of 

 the gill. 



Betv/een the two epithelial layers of the mantle, 

 there is a reticular not-v;ork formed of connective tis- 

 sue, -.vith a siiiall amount of muscle and nerve fibres, etc. 

 The spaces so formed are lacunar blood spaces which are 



