46 



THE CRUSTACEA 



Seme-Organs. — The compound eyes are present in all Branchio- 

 poda. In the Anostraca they are elevated on movable peduncles, 

 but in all other Branchiopoda they are sessile, preserving, however, 

 a certain degree of mobility owing to the fact that they are sunk 

 below the surface and covered by an invagination of the outer 

 cuticle, the cavity of the invagination generally remaining in 

 communication with the exterior by a pore (Fig. 29). Except in 



^ g the Anostraca, the two eyes 



are closely approximated ; 

 and in some Conchostraca, 

 and more completely in the 

 Cladocera, they fuse together 

 into a single eye. The cornea 

 is not distinctly faceted. 

 The crystalline cone is 

 divided into four parts. The 

 rhabdom is surrounded by 

 five rhabdom -cells, but (in 

 ranchipus) is not distinctly 

 divided into rhabdomeres. 



Except in some of the 

 Cladocera, the nauplius eye 

 ])ersists in the adult of all 

 Branchiopoda (Figs. 18 and 

 Fig, o9_ 29, n.e). It is of the usual 



Diagrammatic longitudinal section through the tripartite StrUCture, and may 



compound eye and associated structures of Ajms. i „„,,„„ 4.„J ,, ;fl, fl,„ ■k^r.;,, 



(After Bernard.) hr, brain; c, external cuticle of "^ COUnecteO W ItU tUe Drain 



head; c.c compound eye; m, muscle for moving j^y three Separate UCrVCS Or 

 compound eye ; n.e, nauplius eye ; p, pore leading - . J- 



into the water-sac covering the compound eye and by a single OUC. In the 



sending a blind diverticulum into the nauplius eye ; /^i j • . • j J • 



s, the water-sac, formed by an invagination of the UlaaOCera It IS reaUCed in 



integument. The pore, p, is in the median line, so „• „ „„ J simnlifiprl in ctrnp 



that, as a matter of fact, a strictly longitudinal '='^^*' **"" feliupinieu in bLl UC 



section xjassing through it would pass between tiie ture and mav be entirely 



compound eyes. n ' 



absent. 

 Frontal Organs. — In Brancliipus a pair of organs, presumably 

 sensor^y, lie on the front surface of the head (Fig. 5, /). Each 

 consists of a large hypgdermis cell surrounded by a group of 

 ganglion cells and connected with the brain. Below this, in 

 Brancliipus, is a group of club-shaped cells containing peculiarl}^ 

 shaped rod -like bodies and connected with nerves. In the 

 Cladocera the first-mentioned frontal organs are vestigial, and 

 appear to be represented by a group of cells supplied by a 

 continuation of the nerve of the median eye. The club-shaped 

 cells, however (the " Nackenorgan" of Leydig), are well developed, 

 but are situated high up on the sides or on the dorsal surface 

 of the head. The pair of nerves supplying them originate from 

 the ventral surface of the brain. 



