Sub-class IV. Malacostraca (Thoracipoda, Woodward). -The Crustacea of this sub-class are distinguished by the possession of a generally definite nnmber of body-segments; seven somites going to make up the thorax, and an equal number entering into the composition of the abdomen (counting, that is, the telson as a somite). The Malacostraca are divided into two primary divisions, termed respectively the Edriophthal-mata and the Podophthalmata, according as the eyes are sessile or are supported upon eye-stalks.
This division comprises those Malacostraca in which the eyes are sessile, and the body is mostly not protected by a carapace. It comprises the three orders, Laemodipoda, Isopoda, and Amphipoda. The eyes are generally compound, but sometimes simple, and are placed on the sides of the head. The head is almost always distinct from the body, and the mandibles are often furnished with a palp. Typically there are seven pairs of feet in the adult, hence this division is called Tetradecapoda by Agassiz. In certain Isopods (Tanais) alone is there a carapace.