II. Organs. 3. Alimentary Canal. T. Vascular System. 26 



they are always surrounded by food in a fluid or semi-fluid condition 

 (Cestoda, Acanthocephala) . 



Unlike plants, animals cannot feed upon inorganic substances. In 

 addition to water, tbeir food consists of other organisms, animals and 

 plants ; or of substances derived from them. 



7. Vascular System. 



In many of the lower animals, the digested food stuffs, aftei' 

 traversing the walls of the alimentary canal, make their way 

 through the various tissues by a kind of endosmosis.* In most, the 

 arrangement is, however, somewhat more complicated; there is a 

 special system of branching canals which conveys the nutritive 

 material derived from the gut all over the body, to be absorbed 

 in part by the tissues, after having undergone certain changes 

 within the vessels. This arrangement of tubes is termed the 

 vascular system. 



The vascular network is sometimes very complete, the ultimate 

 "branches ramifying throughout almost all the organs and tissues : 

 usually, epithelia alone are non-vascular, deriving their nutriment 

 from the adjacent tissues. The vascular system may, on the other 

 hand, consist of relatively few definite channels which communicate 

 directly with the spaces of the body. In a well - developed 

 vascular system, some of the vessels are distinguished by their 

 greater width, and from these main trunks, others are given ofE 

 to supply the various regions of the body. These latter, again, 

 give off branches dividing into smaller and smaller twigs, and 

 finally terminating in a network of the finest vessels, traversing 

 the organs. The fluid flows from the larger into the smaller 

 trunks, and thence into the smallest of all, from which it is 

 conducted back into the large trunks by another set of vessels 

 which are also in communication with these minute ones. In 

 most cases there is, therefore, a circulation of the vascular 

 fluid or blood plasma, which is really to be regarded as 

 the digested food, although it must be noticed that it has 

 also received certain waste products of metabolism from the various 

 tissues. It is usually clear and colourless, more rarely coloured red 

 or green. Floating in it are numerous free cells, the blood 

 corpuscles, which are usually amoeboid and colourless ;t less 

 frequently some of the corpuscles are of definite form (unable to 

 throw out pseudopodia) , and red in colour. These red corpuscles, 

 which have the form of circular or oval discs, occur especially 



* In many the distribution of the products of digestion is facilitated by the 

 branching of the alimentary canal, which is furnished with numerous diverticula 

 of considerable extent {e.g., MedussB, Liver-flukes). 



f Their protoplasm may, however, contain coloured granules. 



