The Collaborative International Dictionary of English v.0.48:

Shrimp \Shrimp\, n. [OE. shrimp; -- probably so named from its
   shriveled appearance. See Shrimp, v.]
   1. (Zool.)
      (a) Any one of numerous species of macruran Crustacea
          belonging to Crangon and various allied genera,
          having a slender body and long legs. Many of them are
          used as food. The larger kinds are called also
          prawns. See Illust. of Decapoda.
      (b) In a more general sense, any species of the macruran
          tribe Caridea, or any species of the order
          Schizopoda, having a similar form.
      (c) In a loose sense, any small crustacean, including some
          amphipods and even certain entomostracans; as, the
          fairy shrimp, and brine shrimp. See under Fairy, and
          Brine.
          [1913 Webster]

   2. Figuratively, a little wrinkled man; a dwarf; -- in
      contempt.
      [1913 Webster]

            This weak and writhled shrimp.        --Shak.
      [1913 Webster]

   Opossum shrimp. (Zool.) See under Opossum.

   Spector shrimp, or Skeleton shrimp (Zool.), any slender
      amphipod crustacean of the genus Caprella and allied
      genera. See Illust. under Laemodopoda.

   Shrimp catcher (Zool.), the little tern (Sterna minuta).


   Shrimp net, a dredge net fixed upon a pole, or a sweep net
      dragged over the fishing ground.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Opossum \O*pos"sum\, n. [Of N. American Indian origin.] (Zool.)
   Any American marsupial of the genera Didelphys and
   Chironectes; called also possum. The common species of
   the United States is Didelphys Virginiana.
   [1913 Webster]

   Note: Several related species are found in South America. The
         water opossum of Brazil (Chironectes variegatus),
         which has the hind feet, webbed, is provided with a
         marsupial pouch and with cheek pouches. It is called
         also yapock.
         [1913 Webster]

   Opossum mouse. (Zool.) See Flying mouse, under Flying.


   Opossum shrimp (Zool.), any schizopod crustacean of the
      genus Mysis and allied genera. See Schizopoda.
      [1913 Webster]

WordNet (r) 3.0 (2006):

opossum shrimp
    n 1: shrimp-like crustaceans whose females carry eggs and young
         in a pouch between the legs