A small group, the asellids, is found in fresh water. About 30 species are reported from North America. The isopods are peracarid crustaceans characterized by possession of a ventral thoracic marsupium in which the embryos are brooded. The amphipods, which are also found in southeastern freshwater habitats, are closely related to the isopods. The two can, at least in southeastern freshwaters, be readily distinguished by the shape of the body.
Isopods are dorsoventrally depressed whereas amphipods are laterally compressed.
The two genera, Lirceus and Caecidotea , are quite similar and this exercise can be used with either. Species identification requires dissection of the male gonopods but that is beyond the scope of this course. At the supply desk get a specimen of a mature male asellid and take it to your bench. General Morphology Note the strong dorsoventral depression of these animals. Fig 18.8 is a good drawing of Caecidotea but, unfortunately, is not labeled.