XI 



PHYLUM ARTHROPODA 



523 



ant.f 



the ganglion belonging to their segment may be assumed to have 

 atrophied. The antenna is supplied by a nerve (ant. 2) which 

 springs from the cesophageal connective, but which can be traced 

 backwards to the first ganglion of the ventral chain : this fact 

 may be taken as an indication that the antennas are serially homo- 

 logous with the jaws and feet that they are, in fact, metameric 

 or post-oral appendages which have shifted forwards, one on 

 each side of the mouth, thus becoming pre-oral. The nerve of 

 the antennule (ant. 1) also springs from the cesophageal connec- 

 tive, but is traceable forwards to 

 the brain, where it is connected 

 with a special group of nerve-cells. 

 This has been explained by supposing 

 that the antennule is a post-oral 

 appendage the ganglion of which has 

 moved forwards along the ceso- 

 phageal 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 pre-oral appen- 

 dages, belonging, like the prostomial 

 tentacles of Cheetopods, 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 setae 

 which occur on so many parts of the 

 body, and especially as fringes to the 



T -I J . i Fin. 434. Nervous system 



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 transparent cuticle form- 

 ing the cornea, beneath which is a narrow space or water-sac, com- 

 municating with the exterior by a pore, and therefore filled with 

 water. The eye itself is made up of a large number of radially 

 arranged elements called ommatidia (Fig. 435), 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 body (cr. ) : the whole of this portion of the eye serves to 

 efract the rays of light it is the dioptric apparatus, like our own 



th.fi 



of Apus 

 cancriformis. ant.l nerve to an- 

 tennule ; ant.2 to antenna ; br. brain ; 

 gn. 1 4, first four ganglia of ventral 

 nerve-con] ; md. mandibnlar nerve : 

 mx. 1, nerve of first maxilla ; mx. 2, of 

 second maxilla ; oes. con., cesophageal 

 connective ; op., optic nerve ; th. f. 1, 

 nerve of first thoracic* foot ; v gn. 

 visceral ganglion. (After Lankester 

 and Pelseneer.) 



