STRUCTURE OF CRAYFISH 



325 



C.G- 



O.R 



SOG 



:^ ^ 



V.G 



It is likely that some of the crowded mouth -parts, e.g. the first 

 maxillEe, are almost functionless. The hard toothed knob which forms 

 the greater part of the mandible is obviously well adapted to its crush- 

 ing work. 



In connection with the skeleton, the student should 

 also notice the beak (ros/mm) projecting between the 

 eyes ; the triangular area {epistoma) in front of the 

 mouth, and the slight upper and lower lips ; and the 

 lateral flaps of the body wall which project the gills. 

 Each posterior segment consists of a dorsal arch 

 (tergmn), side flaps (pleura), a ventral bar {sternum), 

 while the little piece between the pleuron and the 

 socket of the limb is dignified by the name of epimeron. 

 The hindmost piece {telson), on which the food canal 

 ends ventrally, is regarded by some as a distinct 

 segment. The most difficult fact to understand 

 clearly, is that the cuticle of certain appendages (e.g. 

 the mandibles), and of the ventral region of the thorax, 

 is folded inwards, forming chitinous " tendons " or 

 insertions for muscles, and, above all, constituting the 

 complex, apparently, but not really, internal, " en- 

 dophragmal " skeleton of the thorax, protecting the 

 ventral nerve-cord and venous blood sinus. 



Muscular system. — The muscles are 

 white bundles of fibres, which on minute 

 examination show clearly that transverse 

 striping which is always well marked in 

 rapidly contracting elements. The muscles 

 are inserted on the inner surface of the cuticle, 

 or on its internal foldings [apodemata). The 

 most important sets are — (i) the dorsal 

 extensors or straighteners of the tail ; (2) the 

 twisted ventral muscles, most of which are 

 flexors or benders of the tail, which have 

 harder work, and are much larger than their 

 opponents; (3) those moving the appendages; 

 (4) the bands which work the gastric mill. 



Nervous system. — The supra-oesophageal 

 nerve-centres or ganglia, forming the brain, 

 have been shunted far forward by the growth 



Fig. 172. — Central nervous system of the crayfish. 



C.G., Cerebral or supra-oesophageal ganglia ; O.R., circum- 

 oesophageal nerve-ring; S.O.G., sub-CEsophageal nerve 

 ganglia, six pairs fused ; V.G., a pair of ventral ganglia ; 

 S.A., the sternal artery passing between the two halves of 

 the ventral nerve-cord ; N., nerves coming from a ventral 

 ganglion. 



N 



