Online Encyclopedia

ENTOMOSTRACA

Online Encyclopedia
Originally appearing in Volume V09, Page 656 of the 1911 Encyclopedia Britannica.
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ENTOMOSTRACA. This zoological term, as now restricted, includes the Branchiopoda, Ostracoda and Copepoda. The Ostracoda have the body enclosed in a bivalve shell-covering, and normally unsegmented. The Branchiopoda have a very variable number of body-segments, with or without a shield, simple or bivalved, and some of the postoral appendages normally branchial. The Copepoda have normally a segmented body, not enclosed in a bivalved shell-covering, the segments not exceeding eleven, the limbs not branchial. Under the heading CRUSTACEA the Entomostraca have already been distinguished not only from the Thyrostraca or Cirripedes, but also from the Malacostraca, and an intermediate group of which the true position is still disputed. The choice is open to maintain the last as an independent subclass, and to follow Claus in calling it the Leptostraca, or to introduce it among the Malacostraca as the Nebaliacea, or with -Packard and Sam to make it an entomostracan subdivision under the title Phyllocarida. At present it comprises the single family Nebaliidae. The bivalved carapace has a jointed rostrum, and covers only the front part of the body, to which it is only attached quite in front, the valve-like sides being under control of an adductor muscle. The eyes are stalked and movable. The first antennae have a lamellar appendage at the end of the peduncle, a decidedly non-entomostracan feature. The second antennae, mandibles and two pairs of maxillae may also be claimed as of malacostracan type. To these succeed eight pairs of foliaceous branchial appendages on the front division of the body, followed on the hind division by four pairs of powerful bifurcate swimming feet and two rudimentary pairs, the number, though not the nature, of these appendages being malacostracan. On the other hand, the two limbless segments that precede the caudal furca are decidedly non-malacostracan. The family was long limited to the single genus Nebalia (Leach), and the single species N. bipes (O. Fabricius). Recently Sars has added a Norwegian species, N. typhlops, not blind but weak-eyed. There are also now two more genera, Paranebalia (Claus, 188o), in which the branchialfeet are much longer than in Nebalia, and Nebaliopsis (Sars, 1887), in which they are much shorter. All the species are marine.
End of Article: ENTOMOSTRACA
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