PHYLUM MOLLUSC A 



191 



gills of the mother, are unable to continue development inde- 

 pendently. Almost without exception the larval mussel must 

 exist for some time as a parasite (Fig. 91 £") on the gills or fins of a 

 fish. During this parasitic existence the glochidia undergo 

 a metamorphosis and finally leave the host (Fig. 91 /^) as juvenile 

 mussels. Glochidia are frequently referred to as "hooked" or 

 "hookless." In the hooked forms, the ventral margin of each 

 shell bears a hinged hook. Glochidia of this type usually become 

 attached to fins or scales of fishes (as in Lasmigona and Strophi- 

 tus). Valves of the bookless glochidia are sometimes provided 



Fig. 91. — Development of fresh-water mussels. A, axe-head type of bookless 

 glochidium of Proptera alata, anterior end view; B, lateral view of same; C, 

 bookless glochidium (Ligumia suhrostrata) posterior end view; D, lateral view of 

 same; E, glochidium of Actinonaias carinata encysted in gill of rock-bass; F, 

 young mussel, same species as C, one week after close of parasitic life, showing 

 lines of growth beyond glochidial shell and protruded foot with its cilia. {After 

 Lef&vre and Curtis). 



with small non-jointed spines (Fig. 91 A) as in the axe-head type 

 or may be entirely devoid of spines (Fig. 91 C and D). Most of 

 the bookless glochidia become attached to the gills of fish hosts. 

 The fingernail shells are hermaphroditic and differ from the 

 other fresh- water Eulamellibranchia in that the young are 

 born alive and do not pass through a larval stage outside the 

 body of the parent. 



In the classification here adopted, the number and arrange- 

 ment of the gills and of the adductor muscles serve as the chief 

 characters for the recognition of orders. Some students of the 

 MoUusca and paleontologists more frequently utilize a system of 

 classification based largely upon hinge structure. 



