CHAP. 11 METAZOA 85 



functions but which, as they go on multiplying by fission, retain 

 the ancestral unspecialized character and, with it the capacity for 

 conjugation. These cells, whose function it is to provide the living 

 substance for subsequent individuals, form collectively what is known 

 as the gonad, while the remaining, much larger, portion of the body forms 

 what is termed the soma. A highly important point to realize about 

 the living substance of the gonad is that so far as we know it is without 

 that great characteristic of most living substance that in due course it 

 dies a natural death. Any piece of the gonad may in the process of 

 syngamy be passed on to a new individual, and this may be repeated 

 so far as we know through an unlimited number of generations, so that 

 the substance of the gonad is potentially immortal. Of course by far 

 the greater part of it is not in practice actually immortal, for it is dependent 

 for its existence upon the soma in which it lives and when this dies it 

 suffers what may be called an accidental death. 



The body of the Metazoon, composed as it is of myriads of cells, 

 reaches a relatively enormous size, and the specialization characteristic 

 of the somatic cells is intimately linked up with needs imposed by this 

 great increase in size. To support the soft semi-fluid living protoplasm, 

 and prevent it from collapsing into a shapeless mass, portions of the cells, 

 or masses of whole cells, are specialized to form hard supporting substance 

 or skeleton. To enable the individual to move, certain tracts of cells 

 in immediate relation to the parts of the skeleton are specialized for 

 contractility, forming the muscles. To deal with impressions from the 

 outer world, to bring about appropriate movements through the muscular 

 system, and in general to control the various living activities, the several 

 regions of the body are linked together by the nervous system. The outer 

 surface of the body — through which the minute Protozoon takes in its 

 nourishment, gets rid of its waste products, and carries out its respiratory 

 exchange of gas with the surrounding medium — becomes hopelessly 

 inadequate to carry out these indispensable functions in the bulky Meta- 

 zoon, and in consequence we find three other important developments. 

 An increase of surface for the taking in of nourishment (and the getting 

 rid of faecal material) is obtained by a part of the surface being pro- 

 longed into the interior of the body in the form of a more or less tubular 

 alimentary canal. A system of finer tubes or vessels arises, through 

 which nourishment and oxygen are distributed to the various tissues of 

 the body and carbon dioxide and other waste products carried from them 

 — ^the blood system or vascular system. Another system of tubular 

 channels are developed, the walls of which have for their special function 

 the extracting of the poisonous waste products from the blood and the 



