THE BIOLOGY OF Ay ANIMAL. 



adult is obvious. It has an elongated, jointed body, traversed 

 by the alimentary canal, which opens in front by the month and 

 behind by the anus. The metamerism is expressed externallv 

 by the jointed appearance, internally by the presence of paired 

 cavities <creloni; separated by dissepiments. Both the body --vail 

 and the alimentary wall consist of two layers: the former of 

 ectoblast without and somatic mesoblast within: the latter : 

 splanchnic mesoblast without (i.e., towards the body-cavity >. and 

 either eutoblast or ectoblast within, according as we consider the 

 rnid-gut on the one hand, or the fore- and hind-gut on the or':. 

 This is shown in Fig. 86, which represents a cross-section of the 

 embryo through the inid-gut. If this be clearly borne in mind 

 the development of all the other organs is easy to understand, 

 since they are formed as thickenings, outgrowths, etc.. of the 

 parts already existing. Fur instance, the blood-vessels make 

 their appearance everywhere throughout the mesoblast. and the 

 nephridia are formed as outgrowths from the dissepiments. The 

 reproductive organs are at first mere thickenings on the somatic 

 layer of the mesoblast. afterwards separating more or less from it 

 so as to lie in the cavity of the coelom. The nervous system is 

 produced by thickenings and ingrowths from the ectoblast. The 

 origin of the different parts is shown in the following scheme : 



TEE GERM-LAYERS AXD THEIR DERIVATIVES. 



The above statements as to the origin of the various organs 

 acquire great interest in view of the fact that they are essen- 

 tially true of all animals above the earthworm, as well as of 

 many below it of all, in a word, in which the three germ-layers 

 are developed, i.e., all those above the Coelenterata or polvps, 

 jelly-fishes, hydroids, sponges, etc. In man, as in the earthworm 



