66 THE CRUSTACEA 



undivided mass in the Cyprididae, but in the Myodocopa it may be 

 more or less completely divided into two or three (Halocypridae) 

 ganglionic masses. 



Sense-Organs. — Paired compound eyes are present only in the 

 Myodocopa (excluding Halocypridae), Avhere they project slightly 

 from the surface of the head within the shell (Fig. 38, 0). The 

 number of ommatidia in each varies from four in Sarsiella to about 

 fifty in some species of Asteropc. The crystalline body in each is 

 bijjartite, and the rhabdom is longitudinally ribbed. There are no 

 facets or cuticular lenses on the overlying cuticle, nor " eye-spots " 

 on the shell. In Philomedes the eyes, though well developed in 

 the male, are rudimentary in the female. 



The nauplius eye is present in the majority of the Ostracods 

 (Fig. 38, 0'), and consists of the usual three pigment-cups contain- 

 ing each from 3 to about 100 retinal rods. In some Podocopa 

 the three parts are widely sej)arated from each other. A uni- 

 cellular lens, or "crystalline body," is found in many Fodocopa 

 (Fig. 12, p. 18), and transparent "eye-spots" on the shell are 

 common. In the Myodocopa the nauplius-eye lies at the base of 

 a rod-like median process from the front of the liead, the " frontal 

 tentacle " (Fig. 38, Stz). Sometimes (Halocypridae) the distal 

 portion of this tentacle is segmented off and is known as the 

 " capitulum." 



Olfactory filaments are present, commonly in greater numbers in 

 the male than in the female, on antennules and antennae, and also 

 on the " brush-like appendage." 



Reproductive System : Female. — The ovaries are paired, and are 

 generally lodged in the posterior part of the body. In the 

 Cyprididae, and partly in the Cytheridae, they lie in the cavities 

 of the shell-valves. The oviducts usually open separately behind 

 the last pair of legs. In the Halocypridae the two oviducts unite 

 shortly after leaving the ovaries, and the unpaired opening is 

 situated on the left side just in front of the caudal furca. A 

 receptaculum seminis, paired except in the Halocypridae, is always 

 present. As a rule, it opens to the exterior by a special copulatory 

 pore, and communicates internally with the oviduct, but sometimes 

 only the external opening appears to be present, while in the 

 Cyprididae the opening into the oviduct serves both for entrance 

 and exit. 



Male. — The testes are simple in the Myodocopa (Fig. 38, 7'), 

 four-lobed in most of the other families. In the Cypridinidae 

 the two vasa deferentia unite to open by a single median pore 

 lying between the paired penes, wliich in this family alone are 

 not traversed by the ducts (Fig. 38, P). In the Halocypridae the 

 penis is unpaired and lies to the right of the middle line. The 

 vasa deferentia unite just before entering it. In the other families 



