CUMACEA. 469 



The Thoracostraca are for the most part marine, and feed on dead 

 animal matter or capture living prey. Most of them are good 

 swimmers ; others, e.g. numerous species of crabs, walk and run and 

 sometimes move sideways or backwards with great agility. The 

 chelae of the first pair of ambulatory legs (fourth thoracic appendages) 

 constitute powerful weapons of defence. Besides the frequent ecdyses 

 of the larval stages, the sexually adult animals cast their shell once 

 01- several times in the year (Decapoda}. They then live with the 

 new ;md still soft skin for some time in protected hiding-places. 

 Some Brachyura are able to live for a long time in holes in the earth 

 a\vay from the sea. These land crabs undertake, usually at the 

 breeding season, common migrations to the sea and return later to 

 the land with their fully developed offspring (Gecarcinus ruricola). 

 The most ancient fossil Podophthalmia hitherto known are the mac- 

 rurous Decapoda and Scltizopoda, from the carboniferous formations 

 (Palteocrangon, Palceocarabus, Pygoceplialus). 



* 



(1) Sub-order: Cumacea." 



TJioracostraca with a small cephalo-thoracic shield, (four to] five 



free thoracic segments, two pairs of maxittipeds, and six pairs of leys, 



of which at least the two anterior jmirs have the biramous tichizopod 



form. The abdomen is elongated and composed of six segments, and 



bears, in the male, two, three or five pairs of swimming feet in addition 



to the caudal appendages. 



The Cumacea, the systematic position of which was formerly very 

 differently estimated, have a superficial resemblance to Decapod larva?, 

 which they also recall in the simplicity of their organization ; while 

 in many of their characters, such as the formation of the brood-pouch 

 and.their embyronic development, they approach the Arthrostraca. A 

 cephalo-thoracic shield is always present and includes, besides the 

 segments of the head, the anterior thoracic segments and their 

 appendages ; the four or five posterior thoracic segments, however, 

 remain free. 



The anterior antenna; are small and consist of a three-jointed basal 

 portion, to the end of which, especially in the male, tufts of olfactory 

 hairs are attached, and of a short flagellum and secondary Hagellum. 



' H. Kriiyer. ' Fire nye Arter afgia?gteii Ouma." .Yaf //>//. TitlxxL'r., Torn II f., 

 1841. H. Krb'yer, " Om Cumaceernes Familie," Xtittirli. 7V</W,'r. X. K.. Tom 

 III., 184<i. G. 0. Sars, " Beskrivelse af cle paa Fregatten Josephines Kxpnl. 

 famine Cumaeeer," Stockholm, 1871. A. Dohrn, ' r Ueber den Ban uiul die 

 Entwickelung der Cumaceen," Jen. nutvi'iciss. Zcitxclu-. Tom V., 1870. 



