COMPARATIVE ANATOMY. 277 



II., which describes the Metazoa. Here, we are told, in contra- 

 distinction to Protista or Protozoa^ we have real Animals or Aleta- 

 zoa. " The bodies of the former consist of one single cell or of 

 several similar cells (with the exception of Vo/vox), each of which, 

 however, is competent to perform all vital functions (cell colony) ; 

 the bodies of Metazoa, on the contrary, always consist of a num- 

 ber of cells, which are not all similar, but have divided among 

 them the different forms of vital activity (cell community). The 

 division of labour may be more or less complete, and according to 

 it the degree of morphological complexity and of physiological 

 perfection is determined. There are animals which are morpho- 

 logically (according to structure) and physiologically (according to 

 their vital activities) only a little raised above the Protozoan 

 colony — e.g., the Hydra. 



" The bodies of these animals consist of only slightly different 

 sorts of cells : digesting cells, neuro-muscular cells, stinging cells, 

 and formative cells of eggs and spermatozoa. All these kinds of 

 cells are, however, indispensable to the existence of the Hydra 

 body ; not one of them can be removed from the body without 

 endangering its existence. The whole body is nevertheless physi- 

 ologically an individual, but, as opposed to the cell, an i?idividual 

 of a second ; i.e., a higher order — a person. Most animals remain 

 at this stage of individuality. A Medusa, a Worm, a Crustacean, 

 or a Mammal, is such an individual of the second order. In many 

 animal divisions, however, the individuals of the second order 

 multiply by fission or gemmation. The new individuals thus aris- 

 ing remain united, and together form i?idividuals of the third order 

 — an animal stock. The single individuals which collectively form 

 such a stock may remain similar, and they may then be related to 

 the stock in just the same way as the cell-individuals of a Proto- 

 zoan colony are related to the colony ; or division of labour again 

 sets in, resulting in variety of development in body, form, and 

 structure of the person forming the stock (polymorphism). Then 

 such r< stock is physiologically again an individual of the third 

 order. The single persons become equivalent to instruments of 

 this complex individual, and bear the same relation to it as the 

 various cell elements of a single individual — e.g., a Hydra — bear 

 to it. As instances of animal stocks without division of labour 



