342 



THE INVERTEBRATA 



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 verte- 

 brata. Sometimes each cup has a lens. In some of the Copepoda the 

 lateral cups are removed from the median one and developed 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 nerve fibrils in their protoplasmic contents. Most of these 

 bristles are branched in various ways and have tactile functions, in- 

 cluding that of appreciating the resistance of the water to movements. 

 In the Decapoda and Syncarida on the basal joint of the antennule 

 (Fig. 227) and in the Mysidae on the endopodite of the sixth ab- 

 dominal appendage there is a pit whose wall bears such hairs while 

 the hollow usually contains sand grains (most decapods) or a cal- 

 careous body formed by the animal (Mysidae). These organs are 





Fig. 226. 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. 



statocysts for the sense of balance. Olfactory hairs or aestheiascs 

 (Fig. 227 B, E) with delicate cuticle stand on most antennules and on 

 many antennae. A pair of groups of cells, sometimes surmounted by 

 setae, standing on the front of the head and known as frontal organs, 

 are found in many crustaceans and are supposed to be sensory. They 

 are present as two papillae in the Naiiplius larva (Fig. 258, ten.). The 

 iiuchal sense organ or "neck organ" of many branchiopods is a group 

 of cells on the upper side of the head containing refractive bodies and 

 connected to the brain by a special nerve. Its function is unknown. 

 As is well known, most crustaceans are pigmented. The pigments 

 are of various colours — red, orange, yellow, violet, green, blue, brown, 

 black, etc., though not all are found in any one species. The majority 

 of them are lipochromcs, though the brown and black are melanins. 

 For the most part they are contained in branched cells (chromato- 



