380 THE CRAYFISH CHAP. 



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-ossophageal ganglion. All the 

 remaining segments have their own ganglia, with the excep- 

 tion 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 oesophageal connectives, and those of the latter from 

 the last abdominal ganglion./ 



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

 tinous cuticle covering the distal end of the eye-stalk is trans- 

 parent, 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^ en- 

 closing a striated body, the rhabdome, and forming the actual 

 visual portion of the apparatus. The ommatidea are opti- 

 cally 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, like 

 the rods and cones of the vertebrate eye (p. 185), covered 



