THOBA.COSTRACA. 463 



of biramous swimming feet, which avi.se from the three posterior free 

 Moments of the tliorax. The thoracic legs are either, at least in part, 

 biramous (with swimming ramus), or as in the Decapods the exopodite 

 is absent and the legs have the form of ambulatory appendages. They 

 then terminate with simple claws : the anterior frequently with large 

 chehe. The terminal joints may however be broad plates, in which 

 case they can be used as swimming feet. The biramous legs of the 

 sixth abdominal segment are. as a rule, broad and fin-like and form, 

 together with the last abdominal segment which is transformed into 

 a large plate (telson), the caudal fin. The feet of the five anterior 

 abdominal segments, on the other hand, are sometimes swimming 

 feet (Stomatopoda), sometimes serve to carry the eggs, or the anterior 

 may assist in copulation (in the male). They may however be more 

 or less rudimentary and some of them absent. 



With rare excep- 

 tions (J///x/'/^) all 

 the Thoracostraca 

 po-->ess gilfe, which 

 are either tufted or 

 composed of regular 

 lancet-shaped leaves. 

 The gills are appen- 

 dages of the liiub> ; 

 in the Stomatopoda 



\'ii'. 363. Cephalothorax of Agtacusfluviafilis, after removal 

 they are attached to O f the branchiostegite (after.Huxley). K, Gills ; B, ros 



the abdominal feet in trum ; < stalked e - ve : M l'> scaphognathite (of the second 



maxilla) ; Mxf", third maxilliped. 



the Sehizopoda and 



Decapoda to the maxillipeds and ambulatory feet. The Cumacea 

 are without gills, except for a single pair on the second pair of maxil- 

 lipeds. In the Decapods they are contained in a special branchial 

 chamber beneath lateral expansions of the carapace (branchiostegite) 

 (fig. 3G3). 



The organs of circulation also attain a high, degree of development, 

 tin- highest not only among the Crustacea, but in general amongst 

 all Arthropods. A heart and vessels are always present. In the 

 Stomatopoda the heart has the form of an elongated tube, which 

 extends through the thorax and abdomen, possesses numerous paired 

 slits, and in addition to an anterior and a posterior aorta gives off 

 to the right and left several branching arterial trunks. In the 

 Ci'niacea, Sehizopoda and Decapoda the heart has a saccular form 

 and lies in the posterior region of the cephalo-thorax. More rarely, 



