102 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



to the round worms, but differ from them remarkably 

 in the mode of development of their body cavity, 

 which is an enterocoele. 



4. Myzostomnin is a form with some points of 

 resemblance to the Chtetopoda ; its characters, how- 

 ever, are still obscure, partly, no doubt, on account of 

 its having taken to the habit of living parasitically 

 on Crinoids, on which alone it has as yet been 

 detected. 



CHAPTER TV. 



ORGANS OF DIGESTION. 



THE activity of a living organism has for one of its 

 chief results destruction and loss of tissue ; this loss 

 can only be made up for by the act of taking in fresh 

 material from the outer world. In the necessary 

 nutrition of an organism, we find that the first 

 process is that of digestion, by means of which 

 substances foreign to the organism become assimi- 

 lated to it, and are rendered capable of being 

 absorbed, and of passing into that stream whence 

 the different parts of a body take, as they require, 

 the food which is needed to make up the losses caused 

 by their several activities. Organisms are, in other 

 words, metabolic. 



It is to be carefully borne in mind that the essen- 

 tial step in the nutrition of an animal is that of 

 assimilation,, and it, indeed, is the only process 

 which obtains in the case of the lowest and simplest 

 organisms. In other words, a simple mass of proto- 

 plasm, such as an Amoeba, takes up from without 

 food material into its own substance, and this, as 

 we have already learnt, is effected directly ; the 

 material thus taken in is acted upon by the living 



