556 



CHAPTER XXX. 



CR USTA CE^ — continued. 

 Sub-class III. — Malacostraca. 



The Crustaceans included in the sub-class Malacostraca (Tlwraci- 

 poda, Woodward) are distinguished by the possession of a generally 

 definite number 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 res- 

 pectively the Hedriophthalmata and the Podophthalmata, according 

 as the eyes are sessile or are supported upon eye-stalks. 



Division A. Hedriophthalmata. 



This division comprises those Malacostraca in which the eyes are 

 sessile, and the body is mostly not protected by a carapace. It 

 comprises the two orders of the 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 thorax, and the mandibles are often furnished with a palp. 

 Typically there are seven pairs of feet in the adult, hence this di- 

 vision has been called Tetradecapoda by Agassiz. 



Order I. Amphipoda. — The members of this order are Crus- 

 taceans, mostly of small size, in which the body is laterally compressed, 

 and the thorax consists of seven segments, carrying seven pairs of legs. 

 The abdomen is mostly well developed, and consists of seven segments. 

 The gills are lamellar or vesicular, and are attached to the basal 

 joints of the thoracic legs. The seven pairs of thoi'acic limbs are di- 

 rected partly forwards and partly backwards. It is from this latter 

 circumstance that the name of the order is derived. 



The order of the Amphipods comprises both marine and fresh- 

 water forms, many familiar types, such as the Sand-hoppers (Talit- 



