276 THE ANATOMY OF INVERTEBRATED ANIMALS. 



ing out a distinction between these two sets of somites, 

 already very clearly indicated by the cervical fold and groove. 



It is for this reason that I differ from Milne-Edwards in 

 regarding the somite which bears the first maxillipedes as the 

 first of the thorax, and not as the last of the head. And the 

 acceptance of this natural delimitation of the head in the 

 higher Crustacea has the advantage of bringing its structure 

 into accordance with that of the same region in the Ento- 

 mostraca, in which it is the rule that the head possesses eyes, 

 antennules, antennae, mandibles, and two pairs of maxillae. 



Another mark upon the carapace is a large and rounded 

 convexity, occupying nearly a third of the whole width of 

 the posterior half of the cephalostegite. This impression is 

 bounded internally by a line drawn from the outer angle of 

 the base of the rostrum, directly backward, and externally 

 by a curved depression, deepening into a pit anteriorly ; it 

 corresponds with the attachment of the base of the adduc- 

 tor muscle of the mandible. 



The mouth of the Crayfish is a wide aperture, situated 

 between the labrum in front, the metastoma behind, and the 

 mandibles on each side. It serves as the entrance to an 

 equally wide oesophagus, a short tube with plaited w r alls, 

 which takes a slightly curved direction upward and a little 

 backward, to open into the large stomach, which is not only 

 situated directly over, but extends forward in front of the 

 gullet. The stomach, in fact, occupies almost the whole 

 cavity of the body in front of the cervical suture, and is 

 divided by a constriction into a large anterior moiety, the 

 cardiac division, and a smali posterior, pyloric portion. The 

 anterior half of the cardiac division has the form of a lar^e 

 membranous bag, the inner surface of which is closely set 

 with minute hairs ; but in the posterior half of this, and on 

 the whole of the pyloric division, the walls of the stomach 

 are strengthened by a very peculiar arrangement of uncalci- 

 fied and calcified plates and bars articulated together, which 

 are thickenings of the chitinous cuticula of the epithelium 

 of the alimentary canal, and constitute the gastric skeleton. 

 The most important part of this apparatus is that which is 

 developed in the posterior cardiac region. 



It consists, in the first place, of a transverse, slightly 

 arcuated cardiac plate (Fig. 73, ca), calcified posteriorly, 

 which extends across the whole width of the stomach, and 

 articulates at each extremity by an oblique suture with a 

 small curved triangular antero-lateral or ptcrocardiac (pi) 



