STRUCTURE OF BODY- WALL 311 



In the Crayfish there is a marked differentiation of the 

 appendages. Those of the prostomium are a pair of eye- 

 stalks, and one of the small feelers or antennules which 

 perform 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 maxillipedes), and five pairs of legs, the first of 

 which are in the fresh-water crayfishes and 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, constitute the tail-fin. 



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

 epithelium (Der. EptJuri) secreting a thick cuticle (Cu), a 

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

 a very thick layer of large and complicated muscles (J/), 

 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. 308), is a cuticular exoskeleton, forming a 

 continuous investment over the whole body but discon- 

 tinuously calcified. 



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

 in the segment 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 (Gul\ a large 



1 The antennules are frequently considered as belonging to the first 

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



