THE OUTER AND INNER TISSUES OF ANIMALS. 283 



288. The early establishment of this primary contrast of 

 tissues answering to this primary contrast of conditions, is no 

 less conspicuous in aggregates of the second order. The 

 feebly-integrated units of a Sponge, with individualities so 

 little merged in that of the whole they form that most of 

 them still retain their separate activities, nevertheless show 

 us, in the unlikeness that arises between the outermost layer 

 and the contained mass, the effect of converse with unlike 

 conditions. This outermost layer is composed of units some 

 what flattened and united into a continuous membrane a 

 kind of rudimentary skin. 



Secondary aggregates in which the lives of the units are 

 more subordinate to the life of the whole, carry this dis 

 tinction further. The leading physiological trait of every 

 creleiiterate animal is the divisibility of its substance into 

 endoderm and ectoderm the purt next the food and the part 

 next the environment. Fig. 147, rudely representing a por 

 tion of the body- wall of a Hydra seen in section, gives some 

 idea of this fundamental differentiation. The creature con 

 sists of a simple sac, the cavity of \vhich is in. direct commu 

 nication w f ith the surrounding water ; and hence there is but 

 little unlikeness between the outer and inner layers : indeed 

 they are said to be capable of exchanging their functions. 

 The essential contrast is that between the parts in contact 

 with foreign substances and the parts sheltered from them 

 between the developed surfaces of the endoderm and ectoderm, 

 and that intermediate stratum of nucleated sarcode from 

 which the two grow in opposite directions. 



Between this case and the case of the Sponge, we may 

 readily trace the connexion. Suppose a mass of Awceba-form 

 units, the outermost of which are united into a layer analogous 

 to that by which a living Sponge is covered, to be represented 

 by a lump of plastic clay ; and for convenience of identifica 

 tion, suppose the surface of the clay to be coated by an 

 extensible film, say of caoutchouc. Let this clay, so coated, 

 be moulded into the shape of a cup ; the cup be gradually 



