STRUCTURE OF THE LAMELLIBRANCHIATE GILL. 297 



Lamellibranchs, from one-tenth to one-half an inch long. 

 They may often be found in abundance near the surface 

 of the mud at the bottoms of stagnant pools and ditches, 

 and sometimes in running water. They are also frequent- 

 ly found climbing upon various water plants. They may 

 be collected by washing the surface mud through the 

 meshes of a fine wire net or strainer. 



If a full-grown Cyclas be carefully opened in a watch- 

 crystal full of water, its gills will usually be found to con- 

 tain from four to ten or twelve embryos in various stages 

 of development. 



The largest embryos are very much like the adults in 

 structure, and their gills are fully formed. They are, 

 therefore, of no use for the present purpose, but they 

 should be carefully studied, as familiarity with their 

 appearance will facilitate 

 the search for smaller ones. 



es. 



FIG. 149. View of right side 

 of a young Cyclas embryo, mag- 

 nified about two hundred diame- 

 ters. (Drawn from nature by W. 

 K. Brooks.) 



s. The two valves of the cal- 

 careous shell, es. The embryonic 

 shell, m. The mantle, mo. The 

 mouth. /. The foot. g. The 

 pedal ganglia, rji. The gill ten- 

 tacles. 



rrio 



FIG. 149. 



a. If one in which the two calcareous valves of the 

 shell have just made their appearance, as a pair of nearly 

 circular patches upon the sides of the embryo, be placed 

 upon a glass slide in a drop of water, and examined with 

 a microscope, the following points may be noticed : 



1. The large, projecting, ciliated foot (Fig. 149, /), 

 indicating the ventral surface of the animal. 



