86 ZOOLOGY FOR MEDICAL STUDENTS chap. 



passing them away to the exterior — the renal or nephridial system. 

 And finally there exist in the bodies of most Metazoa hosts of permanently 

 mobilized amoebocytes — cells which retain a more or less amoeboid 

 character, which are able to creep about actively and to attack, and either 

 destroy or transport to a position in which they are harmless, noxious 

 particles which have found their way into the body such as for example 

 invading microbes. 



The individual cell of the Metazoon, just as the whole individual 

 Protozoon, may be said to be aquatic in its habit, for in order to live it 

 has to be in contact with watery fluid. The whole body then of the 

 Metazoon is permeated, all its intercellular chinks are filled, by watery 

 fluid, forming an internal medium just as the water forms the external 

 medium for a free-living Protozoon. This internal medium however is 

 vastly more complex in chemical composition than ordinary water, for 

 into it are discharged the various products of the metabolism of the living 

 protoplasm. Just as the myriads of cells which constitute the body 

 show obvious specializations of form and structure, so also there exist 

 less obvious peculiarities in their metabolism. Consequently the 

 substances which find their way into the internal medium from the 

 various types of cell are by no means identical but have their own special 

 peculiarities. These various substances, contributed each in its normal 

 proportion, constitute, with the water into which they are discharged, 

 the enormously complicated internal medium of the body. Every living 

 cell in a particular species of animal is adapted to life in an internal medium 

 of specific composition, and \i any particular organ or tissue fails to 

 contribute its quota, or contributes it in abnormal proportion, then the 

 resulting abnormality in composition of the internal medium is apt to 

 have harmful — it may be disastrous — effects on the health of the whole 

 body. 



COELENTERATA 



SCHEME OF CLASSIFICATION 



I. Hydrozoa. 



A. Hydrida. 



B. Hydromedusae. 



(i) Gymnoblastea (Anthomedusae). 

 (2) Calyptoblastea (Leptomedusae). 



C. Acalephae. 



II. ACTINOZOA. 



A. Alcyonaria. 



B. Zoantharia. 



