366 THE CRAYFISH chap. 



brain proper. Hence we have to distinguish between a 

 primary brain or archi-cerebrum — the ganglion of the prostom- 

 ium, 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-asophageal 

 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 gizzard, originating 

 in part from the brain and in part from the oesophageal 

 connectives. 



The eyes 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 onwiatideum, 

 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 optic- 

 ally separated from one another by black pigment, so tliat 

 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 (pp.n) dilates to form an optic ganglion (pp. 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 

 by a layer of nerve-fibres, &c., through which the light must 

 first penetrate. 



Each antennule bears two sensory organs, to which are 



