THE CRUSTACEA 



Sense-Organs. The compound eyes are present in all Branchio- 

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

 but in all other Brarichiopoda 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 

 c e 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 

 persists in the adult of all 

 Branchiopoda (Figs. 18 and 

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



Diagrammatic longitudinal section through the tripartite Structure, and may 

 compound eye and associated structures of Apus. i ' j .> ,1 i 



(After Bernard.) br, brain; c, external cuticle of be connected With the brain 



cotpinmd 3WTJ5faWE 1 ? = BS by three separate nerves or 



into the water-sac covering the compound eye and by a sin"le One. In the 



sending a blind diverticulum into the nauplius eye ; *-*, j . . j , . 



s, the water-sac, formed by an invagination of the LladOCCra it IS reduced 111 



integument. The pore, p, is in the median line, so _ J eimnlifiprl in Qrrnp 



that, as a matter of fact, a strictly longitudinal Slze ana Simplltied m StrUC- 



section passing through it would pass between the ture, and mav be entirely 

 compound eyes. * J 



absent. 



Frontal Organs. In Branchipus a pair of organs, presumably 

 sensory, lie on the front surface of the head (Fig. 5, /). Each 

 consists of a large hypodermis cell surrounded by a group of 

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

 Branchipus, is a group of club-shaped cells containing peculiarly 

 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. 



FIG. 29. 



