ALIMENTARY SYSTEM. 15 



individuality of each unit, for the tailor soon loses the art 

 of making weapons, and vice versa. The greater the extent 

 to which the division of labour is carried the more pro- 

 nounced will be the individuahty of the community. In a 

 similar manner it will be seen that the lower animal types 

 with little physiological division of labour have little in- 

 dividuality and portions of them can survive when separated 

 from the parent, but the higher types have pronounced in- 

 dividuality and death ensues upon the disturbance of a 

 finely-balanced equilibrium of the parts. 



1. The Alimentary System is perhaps the most 

 fundamental ; the parts of which may be divided into : — 



(i) Ingestive System. — The ingestive organs are those 

 connected with the seizure of food and its introduction into 

 the body. As the essential purpose of locomotion is the 

 obtaining of food they are closely aUied to motor organs and 

 are often modified from them. The ingestive aperture is the 

 moiilh^ usually surrounded by organs for seizing or preparing 

 the food, e.g.^ jaws, teeth, tentacles, &c. 



(2) Digestive System. — Digestive organs are more 

 directly concerned with the reduction of food into a soluble 

 and diffusible condition. There is usually a cavity, the 

 enter on or gastric cavity, in which digestion is effected, and 

 there are often digestive glands which secrete a digestive 

 fluid. This cavity is part of the alwtentary canal, occupying 

 the interior of the animal and opening to the exterior by the 

 mouth, or by mouth and anus. 



(3) Egestive System. — Egestive organs are concerned 

 with the removal of waste residue of the food. The egestive 

 aperture when present is called the anus ; in higher types 

 it is usually at the posterior end of the body. 



2. Motor System. — In a motor system the property of 

 contractility is especially concentrated. The primary object 

 of movement is the obtaining of food, and in the case of 

 sedentary (fixed) animals the motor organs are employed, not 

 to move the animal to its food, but the food to the animal. 



The two principal organs of movement are : — 

 (i) Cilia and Flagella. — These are vibratile processes 

 of protoplasm which, by striking the water or surrounding 

 fluid medium, cause either motion of the medium or that of 



