ARTHROSTRAG'A. 



449 



dages closely resemble the typical Phyllopod limb. As a rule, how- 

 ever, some of the anterior thoracic legs take part in preparing the 

 food and have a form intermediate between maxilla? and thoracic 

 legs. Such are called foot- jaws or maxillipeds. In the Arthrostraca 

 the anterior pair of thoracic appendages only are so modified, and the 

 segment bearing them joins the head ; the thorax is, therefore, in 

 this group composed of seven segments, each with its pair of appen- 

 dages. In other groups of Malacostraca the next or two next pairs 

 of thoracic legs have the form of maxillipeds, so that there is no 

 sharp division between the head and thorax. The latter is, at least 

 partially, covered by a shield-like reduplicature of the skin, which 

 morphologically corresponds to the Phyllopod shell and forms a 

 more or less extensive carapace, which fuses with the back of the 

 thorax, and under which the posterior, rarely all the thoracic seg- 

 ments may remain separate as free rings. 



Order 1 . ARTH ROSTR ACA . * 



Malacostraca with lateral sessile eyes, usually with seven, more rarely 

 with six or fewer separate thoracic segments, and the same number of 

 jxiirs of legs. Without a reduplicature of the skin. 



The head bears four antenna?, the two mandibles, four maxillae, 

 and a pair of maxillipeds ; in all six pairs of appendages. A small 

 bilobed plate, distinguished as the under-lip, behind the pair of 

 mandibles, marks the boundary of the primary region of the head. 

 The two pairs of maxillse as well as the maxillipeds are secondary 

 cephalic appendages derived from the thoracic region of the body. 



Behind the head there are usually seven free thoracic rings with 

 the same number of pairs of appendages, which are adapted for 

 creeping or swimming. The number of distinct thoracic segments is 

 in rare cases reduced to six (Tanais] or live (Anceus], the anterior or 

 the two anterior segments of the thorax becoming intimately con- 

 nected with the head. In the latter case a more or less extensive 

 cephalothoracic carapace is formed. The abdomen which follows the 

 thorax includes, as a rule, six segments bearing limbs, and a *imple 

 or split plate without appendages and representing the terminal 

 segment. The number of the abdominal segments and appendages 

 may, however, be reduced (Isopoda), and the entire abdomen may 



* Besides the works of Latreille, M. Edwards, Dana, and others, compare 

 Spence Bate and J. O. Westwood. ' A History of the British sessiloi-yrd 

 Crustacea." Tom. I. and II., London, 1863-1868. G. O. Sars. " Histoire 

 naturelle des Crustaces d'eau douce de Xorvetre," f'hri^tiania, 1SC7. 



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