ENDODERM, MESODERM, AND ECTODERM. 141 



be found on both the external and the internal free sur- 

 faces of the body. So far as these cells belong to the 

 proper external wall of the bod}", they constitute the 

 ectoderm, and so far as they belong to its proper internal 

 wall, they compose the endoderm. Between these two 

 layers of nucleated cells lie all the other parts of the 

 body, composed of connective tissue, muscles, vessels, 

 and nerves ; and all these (with the exception of the 

 ganglionic chain, which we shall see properly belongs to 

 the ectoderm) may be regarded as a single thick stratum, 

 which, as it lies between the ectoderm and the endoderm, 

 is called the mesoderm. 



If the intestine^ were closed posteriorly instead of 

 opening by the vent, the crayfish would virtually be an 

 elongated sac, with one opening, the mouth, affording an 

 entrance into the alimentary cavity : and, round this 

 cavity, the three layers just referred to — endoderm, 

 mesoderm, and ectoderm — would be disposed concen- 

 trically. 



We have seen that the body of the crayfish thus com- 

 posed is obviously separable into three regions — the 

 cephalon or h^ad, the thorax, and the abdomen. The 

 latter is at once distinguished by the size and the 

 mobility of its segments : while the thoracic region is 

 marked off from that of the head, outwardly, only by the 

 cervical groove. But, when the carapace is removed, 

 the lateral depression already mentioned, in which the 



