48 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



The origin, then, of the higher Metazoa is to be 

 looked for in an animal in which an anterior end with 

 a prsestomium is to be distinguished from a posterior 

 end ; in which the two sides are similar to one another, 

 and the dorsal slightly different from the ventral 

 surface. Forms of this kind are still to be found 

 among the lowest Worms. 



Various organs must, of course, be developed with- 

 in such an organism ; in the simplest cases some of 

 the cells of the hypoblast retain the power possessed 

 by the Amoeba of taking solid food into the substance 

 of their own bodies ; the organism being small, no 

 special means of circulating the nutriment thus 

 obtained are required ; and, just as in the Amoeba, 

 respiration is carried on by the general surface of the 

 body, and by the water brought in with the food. On 

 the other hand, even in Amoeba, we found a con- 

 tractile vacuole, and we may, therefore, well suppose 

 that in this complex of cells there must be some 

 special means for the removal from the body of its 

 waste nitrogenous products. At any rate, the meso- 

 blast is on either side channelled by a delicately 

 walled canal which has openings into the spaces in 

 the mesoblast, and communicates by a pore with the 

 exterior. As the organism is to give rise to cells from 

 which other organisms are to arise, some part of 

 its body must be set apart as generative cells ; in 

 the simplest cases these are mere masses of cells in 

 simple pouches, which pass directly into the water. 



Of the cells in the region of the prsestomium 

 some will be more particularly modified for the 

 reception of impressions from the outer world, and will 

 form a rudimentary nervous mass, with which a few 

 nerve-fibres will be connected ; as the creature is 

 capable of moving from place to place, we have, 

 further, to look for the presence of muscular 

 tissue. 



