Leptostraca

Article By:

McLaughlin, Patsy A. Shannon Point Marine Center, Western Washington University, Anacortes, Washington.

Last reviewed:August 2020

DOI:https://doi.org/10.1036/1097-8542.378400

The only extant order of the crustacean subclass Phyllocarida. The Leptostraca is represented by one fossil and a small number of living genera. These malacostracans (see illustration) are unique in having the carapace laterally compressed to such an extent that it forms a bivalvelike shell held together by a strong adductor muscle. The carapace covers only the thorax, leaving exposed the head, with its uniquely movable rostrum, stalked eyes, paired antennules, and antennae. The thorax is composed of eight malacostracan somites, each with a pair of generally similar appendages (thoracopods). Unlike the typical malacostracan abdomen, the leptostracan abdomen consists of seven somites plus a telson with furcal rami. Only the first six abdominal somites carry paired appendages (pleopods). The retention of a seventh somite and caudal furca, in addition to a fully developed telson in the Leptostraca suggested an evolutionarily early offshoot from the malacostracan stem. Although leptostracans are the only extant malacostracans to have seven clearly delineated abdominal somites, evidence from embryological and neurological studies indicates that in some members of the Mysidacea a seventh somite, or the ganglia thereof, remains in developmental stages. See also: Malacostraca

The content above is only an excerpt.