ANNULOSA: CRUSTACEA. 189 



anterior rings are united into a carapace, bearing a pair of 

 larval eyes (ocelli) near the centre, and a pair of large, mar- 

 ginal, or sub- central eyes ; the mouth furnished with a broad 

 post-oral plate, or metastoma, and five pairs of movable appen- 

 dages, the posterior of which form great swimming feet ; the 

 telson, or terminal segment, extremely variable in form ; the 

 integument characteristically sculptured.' (Henry Wood- 

 ward.) 



The Eurypterida are all extinct, and are entirely confined to 

 the Palaeozoic period. Many of them attained to a compara- 

 tively gigantic size; PterygotusAnglicm (fig. 54) being supposed 

 to have reached a length of probably six feet. In their cha- 

 racters they present many larval features ; resembling the 

 larvae of the Decapoda, especially in the fact that all the free 

 somites (except the two anterior ones) were totally devoid of 

 appendages. 





CHAPTER XXXIV. 



MALA COSTS AC A. 



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 Mala- 

 costraca are divided into two primary divisions, termed respec- 

 tively the Edriophthalmata and the Podopkthalmata, according 

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



DIVISION A. EDRIOPHTHALMATA. This division comprises those 

 Malacostmca in which the eyes are sessile, and the body is not 

 protected by a carapace. It comprises the three orders, Lcemo- 

 dipoda, Isopoda, and Amphipuda. 



ORDER I. L^MODIPODA. The Lawwdipoda are small Crusta- 

 ceans, 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 ; 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 Lcemodipoda do not swim, 

 and one section of the order comprises parasitic Crustaceans, 

 of which the Whale-louse (Gijamus Ceti) is the most familiar. 



