GS COMPARATIVE ANATOMY. 



to which they give rise. The idea of a boundary line presupposes a 

 rigidly-defined conception of Animal and Plant. The characteristic 

 of the animal organism may be taken to be that differentiation affects 

 the whole organism. This differentiation consists in its division into 

 two layers, which have been already (§ 28) called ectoderm and endo- 

 derm, and from which the germinal layers of the higher divisions are 

 derived. But the exclusion of all the lower organisms, which do 

 not undergo this division from the Animal Kingdom, would put out 

 of our scope many phenomena which are of great importance, if 

 we would understand animal organisation. Although it might be 

 best to regard this world of lower and very varied organisms as a 

 special Kingdom placed between the Animal and the Vegetable, and 

 containing the beginnings of both, that of the Protista, yet we, 

 as our work embraces the connections between animals and these 

 lowest organisms, must enter into a consideration of them. We 

 therefore unite a number of those divisions of the Protista, which arc 

 more nearly related to animals than plants, as the Protozoa. As 

 their genetic relations to one another are altogether unknown, the 

 division which is formed by these organisms cannot be regarded as 

 a " phylum." Nor is there a type common to them all. We there- 

 fore regard them as lower organisms, which are the nearest of the 

 Protista to Animals, and we must compare them, not with the 

 separate divisions of the higher animal organism, but with them 

 all together. This compels us to unite the latter into a single 

 group, which has been called the Metazoa. 



The Protozoa and Metazoa are not so very sharply marked 

 off from one another. Not a few of the Protozoa are known to be 

 composed of a number of cells. It is the arrangement of cells in 

 layers of definite physiological value which characterises the metazoic 

 organism. This seems to take place very gradually, and at first 

 the layers are incomplete. We find representatives of this in the 

 parasitic Dicyemidas, which live in the so-called veinous appendages 

 of the Cephalopoda, and deserve to be specially mentioned. A 



germinal cell gives rise to a number of cells 

 by division, among which a single large 

 cell becomes surrounded by a number of 

 smaller cells, which form a continuous 

 layer. 



The central cell represents the endo- 



derm, and is covered by the peripheral 



layer, which represents the ectoderm at all 



but one small spot. (Fig. 19.) The endo- 



Fig. 19. Fig. 20. Vcimi- dermal cell elongates considerably, and 



Gasfcrula form embryo of itg protop i asm becomes differentiated in 



stage of Dicyema typus A r T - . , 



Dieycma (after E. van various ways. It iorms the groundwork 



typus. Benecleu). of the elongated body, and remains covered 



by the ectodermal cells, which also grow, 

 without however multiplying very much; these give off fine cilia, 

 and form the protective and locomotor organs of the body, while 



