244 



CUTTLE-FISH, MOLLUSCA, ETC. 



always widest at the bottom, and narrow at the top, 

 which communicates by means of a fleshy syphon or 

 tube with the external water. 



Fig. 176. 



Fig. 174. 





Pholas, in stone. 



Fig. 175 



Sand Mussel (Alya Truncated Sand- 

 arenaria). Mussel (Mya truncata) 



In some bivalves these syphons are double, as in 

 the sand mussels (Mya arenaria and M. truncata) ; a 

 current of fresh sea-water goes down one, and the 

 water, after passing the gills and mantle, and being 

 partly deprived of its organic matter and oxygen, is 

 returned up the other. These mussels burrow in mud 

 and sand, so that little is seen of them except the tips 



