340 ELEMENTARY BIOLOGY. [CHAP. 



Look for specimens in which the ovum is pro- 

 longed into a stalk passing through the above 

 the remnant of an original pedicle of attach- 

 ment to the germinal epithelium. 



y. The germinal vesicle; very large and transparent; 

 it is rounded and usually contains two germinal 

 spots a larger and a smaller one in close appo- 

 sition. 



c. The Glochidiwn larva. If the outer gill appear to 

 be thick and distended, it will be found full of the 

 above. Note, while living, the characters of their 

 shells, the entangled byssus filaments, with which 

 they are provided, and the spasmodic contractions 

 of the adductor muscle. 



Preserve a portion of the larva-laden gill (taken 

 from an animal killed under chloroform) in spirit; 

 when well hardened stain with magenta and exa- 

 mine under a low power. Look for individuals 

 whose valves gape, and note 



a. The larval exoskeleton ; composed of two trans- 

 parent valves, united dorsally by an elastic hinge. 



ft. The shell-teeth ; formed on either side as a spur- 

 like inflection of a. Each terminates in a sharp 

 upwardly directed spike, and its exposed surface 

 is produced into a number of parallel serrated 

 ridges. 



y. The adductor muscle; a powerful transverse band 

 passing between the upper parts of the two 

 valves. 



8. The mantle. This consists, at this stage, of very 

 large cells which project freely into the enclosed 



