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 body, 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 head, 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 
