208 SEA-SIDE STUDIES. 



function of the intestinal canal — from Assimilation, 

 which is the general property possessed by all living tissues. 

 For an animal to grow, and to repair tlie waste which the 

 action of life incessantly produces, it must assimilate, which, 

 as the word implies, means to separate from the external 

 medium such substances as are like to its own substance — 

 or can be converted into them by the vital chemistiy — re- 

 jecting all such as are unlike, or not convertible. Very simple 

 organisms find assimilable food in the element they live in, 

 and the process of separation is easy : they have no stomach, 

 not even a mouth, much less glands secreting solvent fluids. 

 Very complex organisms, on the contraiy, do not, in the air 

 they breathe, or on the earth tlicy tread, find the variety of 

 substances necessary to build up their bodies ; the substances 

 have to be sought, captured, and when found, are not found 

 in an assimilable condition, but in a condition requiring 

 great changes, mechanical and chemical, before they are able 

 to enter into the construction of the tissues. 



An example mil make this plain : Let us first consider 

 the process in the Actinop)hrys, a microscopic animal care- 

 fully studied by Kolliker * It is a mere mass of jelly-like 

 substance, very contractile, without the slightest trace of 

 organs, perhaps also without even a distinct envelope separ- 

 able from the mass.f The outer layer is formed into long 



• SiEBOLD 0. KoLLTKEn's ZtUschrift fur Wissenufhaftliche Zoolofjie, i. ]98. 



t On the existence of a distinct membrane there is much dispute. The 

 arguments, for and against the existence of single-celled animals, are such as, 

 in the present state of inquiry, render decision diflScult. To my mind, however, 

 the researches of A UERBACU {Ueler die Einzelligkeitder Amaben, in SiEBOLD 

 KoLLlKEii's Zcitschrijt, vii. 3t)5) render extremely probable the fact that an 

 enveloping membrane and a nucleus always exist, which afford a veiy strong 



