536 



ZOOLOGY 



SECT. 



cLnt.i 



arU.z 



with the ganglia of the antcnnns in the higher Crustacea. But it 

 is also possible to consider the antennules as pre-oral appendages, 

 belonging, like the prostomial tentacles of Chgetopods, to the 



prostoinial region, and therefore 

 receiving their nerves from the 

 brain or prostomial ganglion. The 

 median and paired eyes are also 

 su23plied by nerves from the brain. 

 Organs of Sense. — The setse 

 which occur on so many parts of 

 the body, and especially as fringes 

 to the limbs, are to be considered 

 as organs of tothcli : the only other 

 organs of special sense are the 

 eyes. The paired eyes are, as we 

 have seen, situated on the dorsal 

 surface of the head, just over the 

 brain : they are covered by trans- 

 parent cuticle forming the cornea, 

 beneath which is a narrow space 

 or water-sac, communicating with 

 the exterior by a pore, and there- 

 fore filled with water. The eye 

 itself is made up of a large num- 

 ber of radially arranged elements 

 called ommatidia (Fig. 428), each 

 of which consists of an outer and 

 an inner portion. The outer portion 

 is a group of clear glassy cells {cc^ 

 enclosing a transparent homogene- 

 ous vitreous hody (cv.') : the whole 

 of this portion of the eye serves to 

 refract the rays of light — it is the 

 dioptric apparatus, like our own 

 The inner portion is a group of 

 retimtla (re), and enclosing a re- 

 the retinula is the actual per- 



th./.t <3 



Fio. 427. — Nervous system of Apus 

 cancriforxnis. ant.' nerve to an- 

 teniiule ; <nit." to antenna ; br. brain ; 

 ffn. 1~U, first four ganglia of ventral 

 nerve-cord ; md. mandibular nerve ; 

 "inx. 1, nerve of first maxilla ; mx. S, of 

 second maxilla ; ces. con., oesophageal 

 connective; o]>., optic nerve ; th. f. 1, 

 of first thoracic foot ; v on. visceral 

 ganglion. (After Lankester and 

 Pelsencer.) 



lens and vitreous humour, 

 sensoiy cells, constituting a 

 fractive rod, the rliabdome (rh.) : 



cipient part of the ommatidium, its cells being comparable to 'our 

 own rods and cones. The retinuias of adjacent ommatidia are 

 separated from one another by cells full of black pigment (p.), so 

 that each ommatidium is in a state of optical isolation from its 

 fellows, and the whole eye is what is called a compcmnd eye. The 

 optic nerve springing from the brain dilates into an optic ganglion, 

 from which fibres pass to the retinulae. 



The median eye is an ovoid body, and consists of four groups 

 of large sensory cells enclosing a mass of pigmented tissue : it 

 is in immediate contact with the brain, and receives a narrow 



