324 ELEMENTARY BIOLOGY. [CHAP. 



cular segments, as seen in the cut edge of the roof 

 of the hinder portion of b. 



d. Follow the glandular segment forwards; it be- 

 comes constricted anteriorly (in the region of the 

 inter-renal aperture) to give rise to the thi7i-waUcd 

 segnic7it; a short whitish tube, which opens into 

 the extreme anterior end of the pericardial 

 chamber by a small reno-pericardial aperture. 

 {Cf. Sect F. 2. a.) 



F. The heart. 



I. Dissect an AtiodoJita from its shell, and remove the 

 whole ventral portion of the body, well below the 

 pericardium. Pin the upper portion down under 

 water dorsal surface uppermost, and carefully lay 

 bare the pericardial cavity. 



The heart will now be exposed; it is a yellowish 

 transparent sac, probably exhibiting regular contrac- 

 tions, composed of a median and two lateral chambers, 

 these are 



a. the ventricle; a median pear-shaped body, its thick 

 end directed backwards; it embraces the hind 

 segment of the alimentary canal. All parts of the 

 wall of the ventricle do not contract together; but 

 a wave of contraction passes, from one end of it to 

 the other, like the peristaltic contraction of the 

 intestine in one of the higher animals. 



h. The auricles ; one of these will be seen on each 

 side, if the ventricle be gently pushed out of the 

 way: each is a somewhat pyramidal sac, continuous 

 with the ventricle at the apex of the pyramid. 



c. Remove the dorsal wall of the ventricle and of a 

 portion of one auricle. Note the thick sj)ongy 



