10-1 GENERAL TRIXCIPLES OF ZOOLOGY 



cannot re-enter the blood-vessels in the same way, on aeeount of the 

 higher pressure in the capillaries. This o\erllo\v is conducted back to 

 the veins by tlie lymph-\ essels. These begin with lacunx in the tissues, 

 and gradually pass into vessels with definite walls. The lymph-vessels 

 of the digesli\'e tract are particularly important since, during digestion, 

 they become filled with the proteid and laity constituents of the digested 

 food; they are called rhylc-vcsscls, because they contain the (7;y/c, dis- 

 tinguished from ordinary lymph by its milky color. 



Cold- and Warm-blooded Animals. — In connection with the blood- 

 vascular svstem, two expressions are much used but not generally correctly 

 understood, viz., cold-blooded and warm-lilootled — or, more correctly, 

 animals with variable and animals with coiishDil Icuipcraliircs. Under the 

 head of animals with var^'ing temperature (poikilolhcrvu:!) or cold blood 

 are placed forms whose temperature is largelv dependent upon the tem- 

 perature of the environment, rising and falling with it, but usually a few 

 degrees above it. In our climate, where the atmospheric temperature is 

 considerably lower than the temperature of the human body, such animals, 

 for example the frog, feel cold to our touch, since they have a much lower 

 temperature than we. 



Such creatures as maintain aljout the same temperature, under 

 any thermal condition are termed warm-blooded or constant temperaturcd, 

 (idiothcrntal, lio))ioifltlicniial) animals. !Man in summer and winter 

 under the equator and at the north pole, has approximately a temperature 

 of 36° C. (qSi^F.), showing higher temperatures only in fever. In order 

 to maintain a constant temperature during the varving conditions, the 

 animal must have the power to regulate the warmth of its botly, either 

 by limiting the production of heat, or liy controlling its loss. If the en- 

 vironment lie warmer than is suitable for the body temperature, then (he 

 production of heat must lie limited to the smallest quanlilv compatible 

 with the \ital processes; but, if this does not suffice, the loss of heat must 

 be increased ]iy evaporation from the surface, usually accomplished liy 

 active perspiration. If, on the contrary, the environment be cold, then 

 every unnecessary loss of heat must be a^■oided, while the production of 

 heat must be increasctl. It is clear that idiolherm-\-, since it requires 

 complicated apparatus, can occur only in the highly organi.'.etl animals. 



rV. Excretory Organs. 



The excretory organs are tubes or glandular canals which open to the 

 exterior either direclh- or b\- wa\- of an end-gut (cloaca), and conduct 

 substances which ha\e become useless to the bodv to the outside. The 

 presence of a ])lood-\'ascular system or a ctelom or both exercises an 



