no' ELEMENTAR Y BIOL OGY. [CHAP. 



into the alimentary canal. The latter presents a short and 

 wide gullet, a stomach surrounded by hepatic follicles, a long 

 intestine coiled upon itself, in a somewhat complicated 

 manner, and, finally, a rectum, which lies in the middle line 

 of the dorsal aspect of the body, traverses the pericardium 

 and the heart which lies therein, and finally ends in the 

 anus. 



As the mouth^ is below and behind the anterior adductor 

 and the rectum passes in front of and above the posterior 

 adductor, it is clear that the alimentary canal, as a whole, 

 lies between the two adductor muscles. 



Digestion, that is solution of the proteinaceous and other 

 nutritive matters contained in food, is effected in the sto- 

 mach and intestine ; and the nutritious fluid, thus formed, 

 transudes through the walls of the alimentary cavity and 

 passes into the blood contained in the blood-vessels which 

 surround it. This blood is thence carried into a large sinus, 

 which occupies the middle line of the body under the peri- 

 cardium and between the organs of Bojanus (see Laboratory 

 Work 5), and receives the greater part of the blood return- 

 ing from all parts of the body. From this median vena cava, 

 branches are given off to the gills and open into the exten- 

 sive vascular network which those organs contain. From 

 this, again, trunks lead towards the pericardium and open 

 into one or other of the two auricles of the heart, which 

 communicate by valvular apertures with the ventricle. The 

 ventricle gives off two aortic trunks, one of which, the ante- 

 rior, runs forwards in the middle line, above the rectum, 

 while the other runs backwards, below the rectum. From 

 these two aortas branches are given off which divide into 

 smaller ramifications for the different regions of the body, 

 and for the viscera, and finally terminate in channels which 

 answer to the capillaries of the higher animals. 



