THE PHYSIOLOGY OF NUTRITION. 737 



Vertebrates, but is only reflexly stimulated into activity by 

 impulses borne by afferent nerves from some part of the 

 body. It seems most reasonable to suppose that this con- 

 dition is primitive, and that the automatic form of activity 

 is derived. The centre for the movement of the chromato- 

 phores is in the sub-cesophageal mass. The activity of the 

 heart is said by some to be purely " protoplasmic," but 

 co-ordination of the parts of the heart, the branchial hearts, 

 &c., is effected by means of the ganglia placed in the 

 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 hectocotylised arm of the male. 



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, &c.), 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 the familiar case of the 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 indigestible residue is rejected. Primarily, this 

 process differs from that found in Vertebrates in that it is 

 zVzMs-cellular, instead of being the result of the action of 

 extra-cellular 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 



47 



