148 



BULLETIN OF THE BUREAU OF FISHERIES. 



f 



Fig. 13. — Glochidia of common fresh-water mussels. (After Surber, 1912 and 1915.) 



a, Strophitus edentulus . 



fe, SympJiynota complanata. 



c, Symphynota compressa. 



d, Symphynota costatn. 



e, Truncilla sulcata. 



f, Tntogonia tubcrculata. 



g. Unto crassidens. 

 h. Unto gibbosus. 



would make a line i inch in length. Hookless glochidia are possessed by practically all 

 of the more important commercial mussels; in fact, as far as we know, this type of glo- 

 chidium characterizes all the genera and species not mentioned in the paragraphs im- 

 mediately preceding and following. 



(3) The "ax-head" type (PI. XIV, figs. 6 and 7) is considered more closely related to 

 the hookless than to the hooked type, although glochidia of this type, except those of 

 a single species, Lampsilis (Proptera) IcEvissima (Coker and Surber, 191 1), possess four 

 hooklike prongs, one at each lower comer of the shell. These pointed projections of 

 the shell are not comparable to the pivoted hooks of glochidia of the hooked type. The 

 ax-head type of glochidium occurs with the following species: LampsUis {Proptera) 

 (data, Imvissima, purpuraia, and capax. (See also text fig. 11, a to f.) 



When the glochidia are fully developed they are ready to break out from the egg 

 membrane and to be liberated from the gills of the mussel, although as previously indi- 

 cated many species of mussels retain the developed glochidia in their gills for many 

 months. A characteristic feature of the mature and healthy glochidium is the active 

 snapping together and opening of the shell. This action can be stimulated by adding a 

 drop of fish blood or a few grains of salt to the water in which the glochidia are held. 



STAGE OF PARASITISM. 



After the fully matured glochidium has been expelled from the brood pouch of the 

 mother, its continued development is dependent upon its coming in contact with the 

 gills or fins of a suitable fish host and attaching to them. If it fails to make this attach- 



