1 84 THE CRUSTACEA 



region and the slender and very mobile abdomen carrying at its tip 

 the styliform uropods (Fig. 111). The extreme specialisation of 

 the respiratory system, Avith the concomitant modification of the 

 anterior part of the carapace, are the most striking features 

 differentiating this from the neighbouring orders. As in the 

 Mysidacea and Tanaidacea the first thoracic limb (maxilliped) 

 carries a backwardly directed membranous epipodite lying in a 

 cavity between the carapace and the side wall of the body ; but in 

 the Cumacea this epipodite is of relatively great size, and is 

 usually (though not always) furnished with respiratory processes or 

 lamellae (Fig. 112, Jr), which may be very numerous and ai'e often 

 better developed in the more active males than in the females. 

 Anteriorly, the branchial cavity is continued as a narrow channel 

 covered by a forward extension of the lateral plate of the carapace, 



Fig. 111. 



Diastylis Goodsiri, 9, from the side, x 4. a', antennule ; ?. i, l^, first and fifth legs (fourth 

 and eighth thoracic appendatces) ; m, bi'ood-poncli ; ps, pseudoi'ostrum ; t, tel.son ; ur, uropod. 

 (After Sars.) 



the two lateral plates generally meeting each other above in front 

 of the head and forming a more or less prominent pseiidorostrum (jjs) 

 divided by a Y-shaped fissure (Fig. 112, /r). Within this channel 

 lies the exopodite of the same appendage in the form of a narrow 

 stalk bearing distally a membranous expansion, which is rolled 

 upon itself to form a tube, or unites Avith its fellow of the opposite 

 side in a single tube (ex) capable of protrusion from the front of 

 the head below the pseudorostrum. Sometimes the pseudorostral 

 plates do not quite meet in front of the head, and in certain genera 

 {Zygosiphon) these plates, and the respiratory channels Avhich they 

 cover, are placed wide apart at the sides of the broadly exj^anded 

 frontal region, Avhile the exopodal tubes project as long transparent 

 siphons. 



Eyes are altogether deficient in many genera. When present 

 they are usually coalesced to form a single median organ (Fig. 

 112, e) borne on the front of the head, Avhich is produced into an 

 oculiferous lobe lying between the two plates of the pseudorostrum. 



