AND THEIR FUNCTIONS. 153 



Aristotle, is a flat cartilaginous fish but has lateral gill- 

 slits. His statement about the gill covers of the fishing-frog 

 is substantially correct. No opercula, such as those of the 

 bass or perch, are present. The respiratory water currents, 

 on each side, flow to the exterior through a large bag-like 

 chamber, bounded exteriorly by a skin-like flap and com 

 municating with the external water through a round hole 

 just behind the base of the pectoral fin. Six very long 

 branchiostegal rays support the skin-like flap. 



Aristotle noticed the existence of gills in some of the 

 invertebrates, e.g., he says that gill-like organs, rough, 

 numerous, and constantly moving, are present in lobsters 

 and crayfishes,* and it is probable that the hair-like organs 

 in the bodies of certain cephalopoda, t and the hair-like 

 organs in some molluscs,! are intended to be gills. 



In his Entoma, Aristotle considered the Jiypozoma, i.e., 

 the part of the body separating the thorax from the 

 abdomen, to be the equivalent of lungs or gills, and to be an 

 organ of smell. 



D. LIVER, SPLEEN, AND PANCREAS. 



Aristotle s statements about the functions of the liver and 

 spleen are few and of very little value; about the functions 

 of the pancreas he says nothing. The liver, he says, cannot 

 be the most important organ of the body nor the origin of 

 the blood, for it does not occupy an important or controlling 

 position, and, further, it is counterbalanced, as it were, by 

 another organ, viz., the spleen. || He was disposed to regard 

 the liver and the spleen as resembling each other in cha 

 racter and constituting a double organ. U Both the liver 

 and the spleen, he says, assist in the digestion of food, by 

 means of their heat, and the spleen withdraws superfluous 

 matters from the stomach and entirely digests them.** 

 Plato s views on the function of the spleen bear some 

 resemblance to Aristotle s, for he says that the spleen, acting 

 like an absorbent body, serves to receive impurities from 

 the liver. 1 1 The liver, Aristotle says, assists in keeping the 

 body in a healthy condition, for this depends very much 



* H. A. iv. c. 2, ss. 7 and 10 ; P. A. iv, c. 8, 684a. 

 f H. A. iv. c. 1, s. 12. | Ibid. iv. c. 4, s. 12. 



P. A. ii. c. 16, 6596. || Ibid. iii. c. 4, 666a. 



IT Ibid. iii. c. 7, 6696. ** Ibid. iii. c. 7, 670a and b. 



ft Timceus, 72, 



