42 



(lin'de to the Mollusca. 



prolonged siphons, but merely a fringed opening in the mantle 

 One very remarkable species, Thecalia concamerala (Case 161), 

 has an internal cup-like proeess within the valves, which serves 

 as a nursing-pouch for the young. 



The freshwater Mussels (Unionidae, Cases 166, 167) are an 

 enormous group of over 1,400 living species. They are found in 



Fig. 32. 



A. Unio graniferus. 





m ; 



B. Anodonta anatina. 



rivers and streams in most parts of the world, but are most 

 plentiful in North America, where they are cultivated for com- 

 mercial purposes (button -making, &c). Margaritana margariti- 

 fera, which is found in the rivers of this country and of Europe, 

 sometimes produces handsome pearls, for which it has been valued 

 since Roman times. Various types of incubating chambers are 

 developed in the gills. 



The young Mussels of this family escape as larvae known as 

 Glochidia, in which each valve of the shell has a hook at the edge. 



