CRUSTACEA 



305 



tocerebrum),^ and for the antennules {deuto- or mesocerebrum) . Except 

 in the Branchiopoda it also contains the antennal ganglia {trito- or 

 metacerebnim). A visceral ("sympathetic ") system is present. 



Sense organs are well developed in the free members of the group. 

 Eyes are of two kinds, the compound eyes, of which a pair is usually 

 present except in the Copepoda and adult cirripedes, and the median 

 eye. Details of the structure of the compound eyes have been given 

 above (p. 274). They may be sessile or stalked, and the latter con- 

 dition has given rise to a theory that they represent a pair of append- 

 ages. Since, however, there are no somites corresponding to their 

 ganglia and since at their first appearance in the embryo they are 

 sessile, this view is not generally accepted (see also p. 277). The 

 median eye (Fig. 215) is the eye of the Nauplius larva, and it 



Fig. 215. A horizontal section through the median eye of Cypris. After 

 Claus. a, position of the median (ventral-anterior) cup, which is not in the 

 plane of section; Is. lens; n. nerve fibres; pig. pigment layer; rd. visual rod; 

 ret. retinal cells. 



persists in most adults, though it is generally vestigial in the Mala- 

 costraca. It consists of three pigmented cups, one median and two 

 lateral, each of which is filled with retinal cells whose outer ends are 

 continued as nerve fibres. Thus the sense cells are inverted, as in the 

 eyes of vertebrata. Sometimes each cup has a lens. In some of the 

 Copepoda the lateral cups are removed from the median one and de- 

 veloped as a pair of lateral eyes. Senses other than sight are subserved 

 by various modifications of the bristles which exist on the surface of 

 the body and contain nerv^e fibrils in their protoplasmic contents. 

 Most of these bristles are branched in various ways and have 

 tactile functions, including that of appreciating the resistance of the 

 water to movements. In the Decapoda and Syncarida on the basal 



^ As in other arthropods, the name procerebrum is given to the anterior 

 part of the brain, composed of the protocerebrum, the optic lobes, and some- 

 times other ganglia which are not connected with paired limbs. 



