These are parasitic forms and very degenerate. ^Sacculina^ lives on crabs, and its term of life is about three years, during which period the afflicted crab does not moult. Its shape is that of an ovoid sac on a stalk, which it attaches between two segments of the ventral surface of its host. The stalk divides and ramifies in a root-like manner within the body of the crab, from whose vital elements it absorbs its nourishment. The roots spread like a mycelium through the whole crab, even to the claws. ^Peltogaster^ lives on hermit-crabs. [pg257] SUBCLASS =MALACOSTRACA= This subclass comprises highly organized /Crustacea/, usually of considerable size, having the appendages much differentiated, the thorax with eight segments, and the abdomen with seven segments. ORDER =SCHIZOPODA= The name, meaning "cleft-footed," applies to the appendages of the thorax, which are once-branched (/biramous/). Gills, when present, are attached to these feet, and hang freely in the water. A delicate carapace covers the thorax; the abdomen is proportionately very large, often twice the length of the cephalothorax. In /Mysis/ the eggs are carried in pouches under the thorax, giving the common name of "opossum-shrimp" to this small, transparent, phosphorescent crustacean. [Illustration: /Mysis sternolepis./] GENUS ^Mysis^ ^M. sternolepis.^ About one inch in length; translucent; antennæ very long; segments marked at joints with dark spots; the last two segments terminate in a stout spine; telson longer than the sixth segment; the sides are nearly straight and are armed with spines; the extremity is cleft; eyes large and prominent. Found abundantly in winter on the shores of still, muddy bays and sounds, especially among eel-grass. ORDER =DECAPODA= ("/Ten-footed/")