XII] ASTACUS 197 



two pairs of antennae which were originally pairs of appendages 

 situated behind the mouth, whilst the first ganglion of the ventral 

 chain termed the sub-oesophageal sends nerves to all the primary 

 jaws and to the first two pairs of maxillipedes, so that it represents 

 five ganglia fused together. The ganglion which supplies the third 

 maxillipede is separated from the sub-oesophageal ganglion only by 

 a groove, and Huxley considered it a part of the sub-oesophageal, 

 which on this reckoning would -supply all the jaws. Each of the 

 "legs" and of the swimmerets is supplied by a separate ganglion, 

 though the ganglia for the fourth and fifth legs are very close 

 together. The commissures between the ganglia for the third and 

 fourth leg diverge widely to allow the sternal artery to pass between 

 them. 



In higher Crustacea such as the Crabs the coalescence of the 

 ganglia has gone much further so that all the jaws and all the 

 thoracic appendages are supplied by a single ganglion. 



The principal sense organs of the crayfish are the eye and the 

 ear. The eye is a modification of the ectoderm covering the tips of 

 the eye-stalks. It consists fundamentally of a number of pits of 

 ectoderm each of which becomes filled with the secretion of the 

 cells forming its walls. It is termed a compound eye to distinguish 

 it from the eyes found in other Arthropoda which consist of a 

 single pit of ectoderm. Starting from the surface each pit consists 

 of a pair of lens cells which secrete ordinary cuticle rather more 

 vigorously than the surrounding ectoderm. Hence a slight con- 

 vexity of the cuticle is occasioned which acts as a. plano-convex 

 lens to concentrate light on the pit. Below the lens cells is a circle 

 of four cells, each of which secretes on its inner side a mass of clear 

 material the conjoined secretions of the four cells constitute a 

 .clear body termed the crystalline cone. Below these again come 

 a circle of six or seven visual sense cells, each of which develops the 

 characteristic visual rod on its inner side whilst from its basal 

 end a nerve fibre proceeds inwards. The visual rods all coalesce 

 to form a fluted striated spindle called the rhabdome and the 

 group of visual cells is termed the retinula. 



Each eye-pit is surrounded by dense black pigment contained in 

 amoebocytes which wander into this position from the surrounding 

 blood. As a result of this investment of pigment the only light 

 which can penetrate the pit and affect the retinula consists of rays 

 very nearly parallel to the axis of the pit. All slanting and oblique 

 light will be absorbed by the pigment layer, and so the retinula is 



