30 



NATURAL HISTORY OF ARTHROPODS. 



Fig. 39. — Cyprls fusea, enlarged, a'. Antennula. 

 a\ Antenna, c. Telson. rf. Maxill£e. c. Eggs. 

 /". First pair of feet. /2. Second pair of feet. 

 g. Gill. m. Mandible. 



developed. Th^ four remaining appendages are one or two maxillas, and three or four 

 ambulatory limbs. These variations in number are due to the fact that one of the 



members functions as a second maxilla in 

 some forms (e. g. Cypris), while in others, as 

 Cythere, the same member appears as one of 

 the locomotive series. 



The various divisions of the alimentary 

 tract are not well marked. In the anterior 

 portion there is a gastric mill slightly resem- 

 bling that of the lobster and other decapods, 

 but more strongly recalling the somewhat 

 similar structure in the Isopoda. Behind 

 this mill arise a pair of tubes to which has 

 been ascribed a hepatic function. Some 

 forms are without a heart, while in others 

 that organ is short, and has three apertures. 

 The nervous system, so far as it has been 

 studied, consists of supra and infraroesophageal 

 ganglia, and a chain of four thoracic ganglia, 

 from which the ambulatory limbs and genitalia are innervated. 



The food may be either animal or vegetable, the latter being taken as well for the 

 animals entangled in it as for the protoplasm of the plant, for the cellulose is here, as 

 elsewhere, unaffected by the digestive juices. 



The complete protection afforded by the shell confers peculiar habits upon the 

 Ostracoda. They scramble about clumsily but rapidly, when undisturbed. Their heavy 

 shells are somewhat of an impediment to graceful locomotion, while the small antennaB 

 are not strong enough to prevent a kind of wobbling in the gait, which sometimes has 

 a ludici'ous effect. When disturbed they make no attempts at resistance or escape, but 

 the feet and antennas are quickly drawn in, the valves close, and the animal, relying on 

 the protection afforded by its hardened shell, sinks to the bottom. 



The Ostracoda have about the same value, in an economic way, as any other group 

 of Entomostraca, but their comparatively small size and numbers make them less 

 important than the Copepoda. They seem able to maintain themselves when other 

 Crustacea fail to do so. At least it frequently happens that they are the sole crustacean 

 inhabitantL of a small pool, and a great number of Ostracoda usually means a paucity 

 of other forms, while other similarly situated puddles may support a more varied 

 entomostracan fauna. 



The living forms are mostly small, averaging about a twentieth of an inch in length, 

 though a few members of the group are larger, reaching a length of a quarter of an 

 inch. The fossil forms sometimes acquire greater dimensions, many being as large as 

 a good-sized bean, and some even measuring three inches in their greatest dimensions. 



As will be seen from the foregoing, there is no group among the Entomostraca, with 

 the exception of the Siphonostomata, which departs more widely from the regular 

 crustacean type than does this order of Ostracoda. The bivalve shell gives the animal 

 a very moUuscan appearance, and the animals, especially the fossil forms, were long 

 referred to that group. 



There are six well-marked families in the order, the first two of which have the 

 antennae simple, the remaining four families having them two-branched. 



