180 



ZOOLOGY 



amount of yolk in some of the cells than in the others. 

 Eventually a shell is formed, which divides into two valves 

 united by a straight hinge. A spine is formed on the free 

 edge of each valve, and a thread (byssus thread) is secreted 

 from the body. The young at this stage is known as 

 Glochidium (Fig. 165). In the spring Glochidia become 

 free from the mother, and attach themselves to the gills or 

 fins of a fish by means of the spines on the shells. Here 

 they become transformed. The single adductor muscle dis- 

 appears, and becomes replaced by the two characteristic of 



' R B. J. 



TD. -- 



FIG. Ki"). A, advanced embryo of Anodonta: B, free glochidium. /, pro- 

 visional byssus ; s, sbell ; .s7t, books ; .sm, adductor muscle ; so, sense organs ; 

 w, cilia. From Korschelt and Heider's " Embryology." 



the adult; the gills arise, the shell gains its adult char- 

 acter, and becomes free from the fish on which it has lived 

 temporarily as a parasite. 



There are other species of fresh-water lamellibranchs 

 besides the Unionidae. These belong to the Cycladidae, 

 allies of the marine Cardidre or Veneridre. They have 

 much the shape of our common hard-shell clam of the sea, 

 but are much smaller in size, rarely exceeding 15 milli- 

 metres. Cyclas (or Sphserium) is of world-wide distribu- 



