THE OSTRACODA. 251 



that the Silurian Hyrnenocaris and its allies were related to 

 Apus. 



The Osteacoda. — This group contains several genera of 

 both recent and fossil Crustacea, for the most part of very 

 small size, and distinguished by their hard, often calcined, 

 and completely bivalve shell, provided with a distinct hinge. 

 The valves of this shell consist of the lateral moieties of the 

 carapace ; they are commonly unequal and unsym metrical, 

 and present a peculiar ornamentation. The shell-gland is 

 very small. The Ostracoda are also remarkable for the ex- 

 tremely rudimental condition of their abdomen, and for the 

 paucity of their thoracic appendages, which, instead of being 

 foliaceous, are strong and subcylindrical, like the ambulatory 

 legs of the higher Crustacea. 



The cephalic flexure is as well marked as in the highest 

 Crustacea, so that the eye, obscurely divided, and median in 

 Cypris (Fig. 66, A), but double and lateral in Cy there (JB), 

 is situated in the upper part of the anterior region of the body. 

 The antennules and antennas, attached to their respective 

 somites, the sterna of which constitute the anterior boundary 

 of the body, are similar in form and function to ambulatory 

 limbs. The ducts of a peculiar gland open, according to 

 Zenker, at the end of the strong spine with which the an- 

 tenna of Cy there is provided. The labrum is conspicuous, 

 and the mandibles are strong, and possess a well- developed 

 palp. The first maxilla is provided with a large foliaceous se- 

 tose appendage (epipodite ?). The second maxilla in Cythere 

 is represented by the first of the three pairs of ambulatory 

 limbs (Fig. 66, JB, e, e, e) present in this genus. In Cypris, 

 which possesses a second pair of maxillae, there are only two 

 pairs of ambulatory limbs (Fig. 66, A, P, i., n.). The aper- 

 tures of the reproductive organs, provided in the male with a 

 wonderfully complex, horny, copulatory apparatus (described 

 with great minuteness by Zenker), are situated between the 

 last pair of thoracic members and the large caudal hooks. 



Strong adductor muscular bundles pass from one valve of 

 the carapace to the other, and leave impressions discernible 

 from without, the form and arrangement of which furnish 

 valuable systematic characters. 



The alimentary canal of the Ostracoda is provided ante- 

 riorly with an apparatus of hard parts, resembling in many re- 

 spects the gastric armature of the Isopoda, and gives origin 

 to two hepatic caeca. Cypris and Cythere have no heart; 



