xxvii STRUCTURE OF BODY-WALL 315 



an olfactory function and also contain the organ of hearing. 1 

 The metameres of the cephalothorax bear one pair of tactile 

 appendages or antennae, six pairs acting as jaws (mandibles, 

 first and second maxillae, and first, second, and third max- 

 illipedes), and five pairs of legs, the first of which are in 

 the fresh-water crayfishes and in lobsters much larger than 

 the rest. The abdomen bears small fin-like swimmerets on 

 its first five metameres, the sixth bearing larger appendages 

 which, together with the seventh segment or telson, con- 

 stitute the tail-fin. 



Sections show the body-wall to consist of a layer of deric 

 epithelium (Der. Epthm) secreting a thick cuticle (Cu\ a 

 layer of connective tissue forming the Dermis (Derm), and 

 a very thick layer of large and complicated muscles (M\ 

 which fill up a great part of the interior of the body. 



The cuticle (Cu) is of great thickness, and except at the 

 joints between the various segments of the body and limbs, 

 is impregnated with lime salts so as to form a hard, jointed 

 armour. It thus constitutes a skeleton which, unlike that 

 of the starfish (p. 312), is a cuticular exoskeleton, forming a 

 continuous investment over the whole body but discon- 

 tinuously calcified. 



The mouth (MtK) is on the ventral surface of the head, 

 in the segments of the mandibles or first pair of jaws. It 

 has therefore, as compared with the mouth of Polygordius, 

 undergone a backward shifting, the appendages of the first 

 metamere (antennae) being altogether in front of it. The 

 enteric canal consists of a short gullet (Gut), a large 

 stomach (St), and a straight intestine divisible into a short 

 anterior division or small intestine (6*. Jnt) and a long 

 posterior division or large intestine (L. Int) : the latter 



1 The antennules are frequently considered as belonging to the first 

 metamere, the number of segments being then reckoned as twenty. 



