536 



ZOOLOGY 



SECT. 



oes. ccn 



ant.i 



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

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

 belonging, like the prostomial tentacles of Chsetopods, to the 



prostomial region, and therefore 

 receiving their nerves from the 

 brain or prostomial ganglion. The 

 median and paired eyes are also 

 supplied 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 touch : 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 rilled with water. The eye 

 itself is made up of a large num- 

 ber of radially arranged elements 

 called ommatidia (Fig. 428), each 

 FIG. 4-27.-Nervous system of Apus of which consists of an outer and 



cancriformis. ant.' nerve to an- an j nner por ti n. The Ollter portion 

 tennule ; ant." to antenna ; (//. brain ; 11 / \ 



rjn. 1-k, first four ganglia of ventral is a group of clear glaSSY Cells (CC.) 

 nerve-cord ; md. mandibular nerve ; i i 



mx. i, nerve of first maxiiia ; mx. 2, of enclosing a transparent homogene- 



second maxilla \ <, *. con., oasophageal r>Hrcn'll<* hndtl ( CV V thp whole 

 connective; op., optic nerve ; ft./. 1, 



of first thoracic foot ; v gn. visceral o f this portion of the CVC serVGS to 



ganglion. (After Lankester and *- . _ ,. , > . . , 



Peiseneer.) retract the rays of light it is tlie 



dioptric apparatus, like our own 



lens and vitreous humour. The inner portion is a group of 

 sensory cells, constituting a retinula (re.), and enclosing a re- 

 fractive rod, the rliabdomc (rli.) : the retinula is the actual per- 

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

 own rods and cones. The retinula? 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 compound eye. The 

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

 from which fibres pass to the retinula?. 



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 



