CHAP. II INTRODUCTION TO STUDY OF METAZOA 55 



of labour again steps in, resulting in variety of development in body 

 form and structure of tbe persons forming the stock (polymoFphism). 

 Then such a stock is also 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 among the persons we have 

 most Corals ; and of stocks with far-reaching division of labour and 

 polymorphism the SiphoTwphara. 



Even in the lowest Metazoa the cell elements are not found 

 scattered in the body without any special arrangement. On the con- 

 trary, we find even among the simplest Calenterata that they are 

 arranged in two epithelium-like layers, which are closely contiguous and 

 form the wall of the body, which is pouch-shaped and provided with an 

 opening. In keeping with the physiological activities of the various 

 cells, the stinging cells and the neuro-muscular cells form the outer 

 layer, while the digesting cells form the inner layer, which is turned 

 towards the pouch cavity, i.e. the gastric cavity. The reproductive 

 cells lie protected in the deeper portions of the outer layer. These 

 two layers, which occur in the development of all Metazoa, are called 

 the Ectoderm and the Endoderm. 



Either similar or dissimilar cells or tissues, therefore, may combine 

 to form cell or tissue complexes. Such complexes are called organs 

 when the cells or tissues combining to form them perform in common 

 one or more functions. The endoderm of the Hydra is a primitive 

 organ, all the cells of which undertake the digestion. The tentacles 

 of the Hydra are slightly more complicated organs ; they serve as 

 organs of touch, as weapons, and as organs for seizing food. For these 

 purposes they contain neuro-muscular and stinging cells. For the 

 nutrition of the tentacles canals lined with endoderm enter them from 

 the gastric cavity. In this way the most various elements are drawn 

 into the service of one or of several functions after which the organs 

 are named. We therefore speak of the sensory organs and the organs 

 of movement, respiration, etc. 



Several organs of the same sort with similar functions may occur 

 in the same body (this is especially the case among the higher animals) ; 

 these are then portions of an organic system — muscular system, 

 vascular system, nervous system, etc. 



The observation of the construction of the animal body out of cells, 

 tissues, organs, etc., is the object of Anatomy, microscopic and mac- 

 roscopic. These become Comparative Anatomy when the structure 

 of animal bodies is considered comparatively. Comparative anatomy, 

 again, is the indispensable aid of zoology in one of its chief tasks — the 

 discovery of the natural relations of affinity among animals and of the 

 conjectural course of development of the animal world in the earth's 

 history. Comparative anatomy seeks to define the relations of affinity 

 between the different portions which combine to form the animal body. 



