40 



ON THE PLACE OF MAN IN THE SCALE OF BEING. 



degrees above that of the surrounding medium ; but many of them are capa- 

 ble of being subject to extreme variations of heat and cold, without their vita- 

 lity being thereby destroyed. Their respiration is for the most part aquatic ; 

 and is performed by means of gills, over which a current of water is con- 

 stantly being propelled, by the vibration of the cilia that cover their surface. 

 Many of them are dependent on the same current for their supplies of food ; 

 part of the water so introduced being taken into the stomach ; and a part flow- 

 ing over the res^ratory surface. The higher tribes, however, go in search 

 of their food, and have instruments of mastication for reducing it; but in 

 these, as in the former, the anal orifice of the intestine opens into the passage, 

 through which the current that has passed over the respiratory organs finds 

 egress ; so that the faBcal matter from the former, and the fluid that has served 



Fig. 3. 



t- 



Aplysia cut open, showing the viscera; a, upper part of oesophagus; b, pen's; c. c, salivary glands; d, 

 superior or cephalic ganglion ; e, e, inferior, or subcesophageal ganglia ; /, termination of oesophagus ; g. g. first 

 stomach; h, third stomach; i, second stomach; k, intestine; Z, I, Z, liver: 771, posterior ganglion; n, aorta; 

 o, hepatic artery; y, ventricle of heart; g, auricle; r, s, branchiae ; t } testis; w, lower part of intestine ; , 

 ovary ; w, anus. 



