The body of an ostracod is encased by two valves, superficially resembling the shell of a clam. A distinction is made between the valve (hard parts) and the body with its appendages (soft parts). The body consists of a head and thorax, separated by a slight constriction. Unlike many other crustaceans, the body is not clearly divided into segments. The abdomen is regressed or absent, whereas the adult gonads are relatively large. The head is the largest part of the body, and bears most of the appendages. Two pairs of well-developed antennae are used to swim through the water. In addition, there is a pair of mandibles and two pairs of maxillae. The thorax typically has two pairs of appendages, but these are reduced to a single pair, or entirely absent, in many species. The two "rami", or projections, from the tip of the tail, point downwards and slightly forward from the rear of the shell. Ostracods typically have no gills, instead taking in oxygen through branchial plates on the body surface. Most ostracods have no heart or circulatory system, and blood simply circulates between the valves of the shell. Nitrogenous waste is excreted through glands on the maxillae, antennae, or both. The primary sense of ostracods is likely touch, as they have several sensitive hairs on their bodies and appendages. However, they do possess a single naupliar eye, and, in some cases, a pair of compound eyes, as well.