780 COMPARATIVE PHYSIOLOGY. 



course of the visceral nerves and their branches. The 

 arms are very well innervated, containing a central nervous 

 axis ; even a severed arm is said to exhibit powerful reflex 

 movements. This property is probably of some use in the 

 free hecto-cotylised arm of the male. If a " brain " be 

 defined as the general motor centre associated with at 

 least one of the higher sensory nerves, then the Crustaceans 

 and Tracheata have "brains" ; but this can hardly be said 

 of Molluscs or of Annelids. 



THE PHYSIOLOGY OF NUTRITION. 



We have seen that by means of the nervous system the 

 animal is brought into relations with the external world. 

 It is in consequence constantly evolving energy in the form 

 of movement, heat, electrical energy ( Gymnotus, etc.), or light 

 (phosphorescent animals). We now proceed to consider 

 the manner by which this loss of energy is made good, that 

 is the Nutrition of the Tissues. Inasmuch, however, as the 

 food of animals typically consists of very complex organic 

 substances, the process of digestion must first be considered. 

 Digestion is the process by which the organic substances of 

 the food are broken down into simpler substances, which 

 are soluble and diffusible, and capable of being assimilated 

 and built up into the substance of the tissues. 



Digestion. In many of the Protozoa, as in the familiar 

 case of Amoeba, solid food particles are ingested, they are 

 surrounded by fluid, and eventually the fluid is absorbed 

 with the products of digestion, while the useless and in- 

 digestible residue is rejected. Primarily, this process differs 

 from that found in Vertebrates, in that it is /////-^-cellular, 

 instead of being the result of the action of extra-ce\\u\ar 

 ferments. There is some doubt as to whether the Protozoan 

 type of digestion is also due to ferments, or whether the 

 living protoplasm has the power of directly inducing changes 

 in substances brought into contact with it. Krukenberg suc- 

 ceeded in extracting a peptic ferment from the plasmodium 

 of " flowers of tan," but did not believe that it could have 

 a digestive function, on account of the alkalinity of normal 

 protoplasm. Metchnikoff, however, has demonstrated in 

 some cases that the fluid of "food vacuoles" is acid, and 



