334 ELEMENTARY BIOLOGY. [CIIAP. 



lirisms are mostly hexagonal or pentagonal in 

 contour; transverse diameter variable, often in 

 proportion as the section passes through their 

 inner ends. 



€. 



Dissolve out the earthy matter from a piece of 

 a valve, by treatment with weak hydrochloric 

 acid. An organic basis remains behind, conform- 

 able in shape to the fully formed structure. 



L. The study of transverse sections. 



Remove one valve from an animal which has been dead 

 some 6 — 8 hours, and make sections as directed below — 

 cutting through the soft parts with a razor or sharp scalpel 

 and through the remaining valve with a bone-forceps. 

 Examine under water, and if some of the more minute parts 

 to which attention is directed do not fiiU in the plane of 

 section, dissect until they are reached. 



Sections a^ b, r, c, and f to be transverse to the long axis 

 of the body, d to be oblique. 



a. Through the anterior pericardial region, immedi- 

 ately behind the excretory and genital orifices. 

 Work over — 



a. The body ; median, laterally compressed below. 

 Note the muscular niture of the body-wall ; it is 

 thickened ventrally to form the locomotor foot — 

 above the gills it is expanded dorso-laterally and 

 modified to form the orga?i of Kcbcr. 



(i. TYiQpaliiai /obes ; membranous outgrowths of the 

 thickened dorsal region of the body-wall; they 

 are in close apposition with the valves. Note 

 the thickening C)f their free borders to form the 

 pa ilia I viuscies. 



