242 MANUAL OF ZOOLOGY. 



CHAPTER XXXIV. 



MALACOSTRACA. 



SUB- CLASS IV. MALACOSTRACA. The Crustacea of this sub- 

 class 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 respectively the Edriophthalmata and the Podophthal- 

 mata according as the eyes are sessile, or are supported upon 

 eye -stalks. 



DIVISION A. EDRIOPHTHALMATA. 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, Lamodipoda, 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. 



ORDER I. LJEMODIPODA. The Lcemodipoda are small Crus- 

 taceans, which are distinguished amongst the Edriophthalmata 

 by the rudimentary condition of the abdomen. The first 

 thoracic segment is amalgamated with the head, and the limbs 

 of this segment appear to be inserted beneath the head, or, as 

 it were, beneath the throat (fig. 92) ; hence the name given to 

 the order. The respiratory organs are in the form of two or 

 three pairs of membranous vesicles attached to the segments 



of the thorax, or to the 

 bases of the legs. The 

 last pair of feet are either 

 inserted at the end of the 

 last somite, or are followed 

 by not more than one or 



Fig. 92. Ljemodipoda. Caprella phasma. tWO Small Segments. There 



are four setaceous antennae, 



and the mandibles are without palps. The body is generally 

 linear, of eight or nine joints, but is sometimes oval. The 

 feet are hooked. The Lamodipoda are all marine, and one 

 section of the order comprises parasitic Crustaceans, of which 

 the Whale-louse (Cyamus Ceti) is the most familiar. The 



