338 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



mouth and connected therewith by a short oesophagus. 

 If we take off the carapace of a lobster by dividing the 

 attachments with the sternum, Fig. 221 will represent 

 what we see. Immediately beneath the carapace is the 

 heart, H, with the blood vessels running fore and aft. 

 We have nothing to do with these now. We will speak 

 of them later. In front and beneath the heart is seen 

 the large stomach, st, with the straight intestine, i, run- 

 ning backward beneath the great artery. Behind the 

 stomach, on each side of the anterior part of the intes- 

 tine, is seen the large liver, Z, composed of many tubules 

 opening into the intestine. The stomach is very mus- 

 cular, and if opened is seen to contain what might be 

 called stomack teeth — i. e., powerful triturating organs 

 composed of chitin and armed with prickles of the same. 



MOLLUSCA. 



We take one example from each class. 

 Acephala. — Among these we take the Mactra (Fig. 

 222). In all acephala the food-taking is wholly involun- 



Fig. 222. — Mactra with one valve removed, showing the anterior {am) and 

 the posterior {pm) shell muscles, and the foot {/). (From Gegenbaur.) 



tary. The clam buries itself in mud with the mouth 

 downward and the siphon upward, just reaching the 

 water. By ciliary action, currents are created which 

 pass down one tube of the siphon, through the gills, 

 contributing thus to respiration ; then to the mouth, 

 contributing thus to alimentation; then back again 



