202 GENERAL ZOOLOGY 



sent along the dorsal vessel, and force the 'blood into theJ 

 ventral blood-vessel (Fig. 100, 22), which carries it posteriorly 

 in a regular flow. Below the ventral nerve-chain is the sub- 

 neural blood-vessel (Fig. 100, 23), in which the blood prob- 

 ably flows backward. The lateral blood-vessel (Fig. 100, 21), 

 with its connections, is limited to the region anterior to the 

 crop. Capillaries (fine blood-vessels joining larger ones) branch 

 in the wall of the intestine and connect in a very complicated 

 fashion with all the large vessels. Absorbed food passes into 

 the capillaries and is carried with the blood to the larger 

 blood-vessels, to be transported to all parts of the body. All 

 about the intestine, in the hollow spaces of the somites, tliei 

 is a large quantity of body-cavity fluid, which is very much 

 like the blood in the vessels, except that it is colorless. It is 

 thought that much of the absorbed food mixes with this 

 fluid. The outcome is the same with food carried by the regu- 

 lar circulatory system and with that taken up by the bod}-- 

 cavity fluid. 



Assimilation. The food is transported by the blood in the 

 blood-vessels or in the bodj^-cavity to tissues, where some of 

 it is transformed into protoplasm. Just how this is done no 

 one has been able to discover, but it is known that the trans- 

 forming process, which is called assimilation, takes place in 

 tissues which are alive, for example, muscles and nerves. 

 We know that the building up of new protoplasm takes plac 

 in growth, when new cells are formed, and that it is also made 

 necessary on account of the slow and imperceptible destriu. 

 tion of the protoplasm in oxidation (see below). 



It will be helpful at this point to know that of all the food 

 taken into the circulatory system of an animal but very little 

 except during growth, is actually made into protoplasm.^ 

 The carbohydrates and the fats that are absorbed, aiid those 

 that are made from proteids by the protoplasm, together with 

 some unassimilated proteids, are destroyed after they have 



