220 SEA-SIDE STUDIES. 



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 chemistry — reject- 

 ing 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 contrary, do not, in the air they breathe, or on the 

 earth they 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 will make this plain : Let us first con- 

 sider the process in the Actinophrys, a microscopic 

 animal carefully studied by KoUiker.* It is a mere 

 mass of jelly-like substance, very contractile, mthout 

 the slightest trace of organs, perhaps also without 

 even a distinct envelope separable from the mass.-[* 



* SlEBOLD ti. Kolliker's Zeitschrift fur Wissenscliaftliche Zoologie, 

 i. 198. 



■Y 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 difficult. 

 To my mind, however, the researches of Auerbach ( Ueher die Einzel- 

 ligheit der Amoeben, in Siebold ii. KuLLIKEr's Zeitschrift, vii. 365) ren- 

 der extremely probable the fact that an enveloping membrane and a 

 nucleus always exist, which afford a very strong argument in favour of 

 the unicellular structure of these animals. Although I have been un- 

 able^ in repeated examinations, to convince myself of the existence of a 



