CH. X] THE CONDITIONS OF LIFE IN THE SEA 229 



The whole surface of the skin of an internal parasite is bathed in 

 the juices of the host — the blood, if it lives in the lumen of the 

 heart, or a blood-vessel ; the lymph if it inhabits the peritoneal 

 cavity; or the digested food-substance if it lives in the intestine. 

 All these liquids are rich food media, and the parasite simply 

 absorbs the nutriment which they contain over the entire surface 

 of its body. We usually make, or imply, some distinction between 

 the method of feeding of a parasite and that of a closely allied 

 species ; most biologists would assume, for instance, that the mode 

 of nutrition of an Ascaris or Bilharzia was different from that of 

 the free-swimming stage of a Gordius, and that a parasitic 

 Turbellarian like Graffilla differed in a corresponding manner 

 from those species which live in the open. 



Comparative physiology has not received anything like the 

 same amount of study as comparative anatomy ; and so we find 

 that our notions of the metabolism of the lower invertebrata has 

 been profoundly influenced by our extensive knowledge of the 

 physiology of the warm-blooded animals. Morphological studies 

 have enabled us to trace homologous structures and organs through 

 long series of animal forms ; thus the ear-ossicles, and parts of the 

 jawbones of the mammal, are homologous with parts of the 

 visceral skeleton of the fishes ; and the lungs of the warm blooded 

 animal are morphologically the same thing as the swim bladder of 

 the fish. But whereas in these specific cases we recognise quite 

 clearly that the functions of the homologous structures are different, 

 we do not always bear in mind that the functions of morphologically 

 similar structures, in the invertebrata, may be entirely different. 

 So great is the force of well-known words that we naturally assume 

 that the gills of a fish, the gills (ctenidia) of a mollusc, and the 

 gills (respiratory plumes) of a tube-living worm have the same 

 functions, those of taking up oxygen from, and giving ofi" carbonic 

 acid to the sea water. Just in the same manner we assume that 

 the digestive cavity (stomach and intestine) of a fish, the digestive 

 cavity (coelenteron) of a coral polyp, and the digestive cavity 

 (gastric pouches and canals) of a medusa, have similar functions ; 

 those of digesting and absorbing the materials of the food-animals 

 captured. 



So also with regard to our notions of the nature of the 



