334 ELEMENTARY BIOLOGY. [CHAP. 



prisms are mostly hexagonal or pentagonal in 

 contour ; transverse diameter variable, often in 

 proportion as the section passes through their 

 inner ends. 



t. 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 fall in the plane of 

 section, dissect until they are reached. 



Sections a, b, c, e, 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 — 



u.. The body ; median, laterally compressed below. 

 Note the muscular nature 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 organ of Keber. 



j3. The pallial lobes ; membranous outgrowths of the 

 thickened dorsal region of the body- wall; they 

 are in close apposition with the valves. Note 

 the thickening^ of their free borders to form the 

 pallial muscles. 



