494 



ZOOLOGY 



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with a special group of nerve-cells. This has been explained by 

 supposing that the aiitennule is a post-oral appendage the ganglion 

 of which has moved forwards, along the oesophageal connective, 

 and fused with the brain a process which actually takes place 



with the ganglia of the antennae 

 in the higher Crustacea. But it 

 is also possible to consider the 

 antennules as prse-oral append- 

 ages, belonging, like the pro- 

 stomial tentacles of Chastopods, to 

 the prostomial region, and there- 

 fore 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 seta3 

 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 liead, just over 

 the brain: they are covered by 

 transparent cuticle forming the 

 cornea, beneath which is a narrow 

 space or water-sac, communicating 

 with the exterior by a pore, and 

 therefore filled with water. The 

 eye itself is made up of a large 

 number of radially arranged ele- 

 ments called ommatidia (Fig. 

 394), each of which consists of an outer and an inner por- 

 tion. The outer portion is a group of clear glassy cells (ce.) 

 enclosing a transparent homogeneous vitreous . body (cr.) : the 

 whole of this portion of the eye serves to refract the rays of 

 light ; it is the 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 refractive rod, 

 the rliabdome (rh.) : the retinula is the actual percipient parj; 

 of the ommatidium, its cells being comparable to our own rods 

 and cones. The retinulaB 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 



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Fio. 303. Nervous system of Apus 

 cancriformis. ant.' nerve to an- 

 tennule ; ant." to antenna ; br. brain ; 

 rjn. 1 It, first four ganglia of ventral 

 nerve-cord ; md. mandibular nerve ; 

 mx. 1, nerve of first maxilla; mx. 3, of 

 second maxilla ; th.f. 1, of first thoracic 

 foot; r. fin. visceral ganglion. (After 

 Lankester and Pelseneer.) 



