380 THE CRAYFISH CHAP. 



only the eyes and antennules, but the antennae as well : 

 a study of development shows that the ganglia belonging 

 to the antennary segment have fused with it. Hence 

 we have to distinguish between a primary brain or 

 archi-cerebrum the ganglion of the prostomium, and a 

 secondary brain or syn-cerebrum, formed by the union 

 of one or more pairs of ganglia of the ventral cord with 

 the archi-cerebrum. A further case of concrescence of 

 ganglia is seen in the ventral nerve-cord, where the 

 ganglia of the last three cephalic and first three thoracic 

 segments have united to form a large compound sub- 

 cesophageal ganglion. All the remaining segments have 

 their own ganglia, with the exception of the telson, which 

 is supplied from the ganglion of the preceding segment. 

 There is a visceral system of nerves supplying the fore- 

 gut and hind-gut, the nerves of the former originating 

 in part from the brain and in part from the cesophageal 

 connectives, and those of the latter from the last 

 abdominal ganglion. 



The eyes (Fig. 97) have a very complex structure. 

 The chitinous cuticle covering the distal end of the eye- 

 stalk is transparent, is divided by delicate lines into 

 areas or facets which are mostly square, and constitutes 

 the cornea. Beneath each facet of the cornea is an 

 apparatus called an ommatideum, consisting of an outer 

 segment or vitreous body having a refractive function, 

 and an inner segment or retinula, enclosing a striated 

 body, the rhabdome, and forming the actual visual portion 

 of the apparatus. The ommatidea are optically separated 

 from one another by black pigment, so that each is a 

 distinct organ of sight, with a very limited visual area, 

 and the entire eye is called a compound eye. The optic 

 nerve (op. n) dilates to form an optic ganglion (op. gn) 

 in close*connection with the inner ends of the ommatidea ; 

 the latter are thus turned towards the light, and are not, 



