cephalosome

The cephalosome is a term used in zoology to describe the anterior (head) portion of a crustacean or the anterior part of a chelicerate, such as a spider or scorpion. In crustaceans, the cephalosome is formed by the fusion of the head and the first few thoracic segments. This fused structure typically bears the eyes, antennae, and mouthparts. The number of thoracic segments fused with the head can vary among different crustacean groups. For example, in decapods like crabs and lobsters, five thoracic segments are fused with the head.

In chelicerates, the term cephalosome is often used interchangeably with prosoma. This anterior body section is the primary sensory and feeding region. It bears all the appendages that are not part of the opisthosoma (abdomen), including the chelicerae, pedipalps, and the walking legs. Unlike crustaceans, chelicerates do not have antennae. The cephalosome in chelicerates is a distinct tagma, or body region, characterized by its function in locomotion, feeding, and sensory perception. The eyes are typically located on the dorsal surface of the cephalosome. The term cephalosome helps to delineate distinct body regions in these arthropod groups, aiding in their classification and understanding of their anatomy and evolution.