198 THE CRUSTACEA 



young. Very often some or all of the abdominal somites are fused 

 together, and the last somite is always coalesced with the telson 

 except in certain Anthuridae. The abbreviation of the abdomen 

 is least in the Phreatoicidae ; in the other families the anterior 

 somites are short and crowded together, though the telsonic seg- 

 ment is often of large size. The abdominal somites are usually 

 expanded laterally into pleural plates, and the thoracic somites 

 may be similarly produced over the bases of the legs, but in these 

 the pleural plates are often supplemented or replaced by the 

 expanded coxopodites which here form the coxal plates or so-called 

 "epimera." 



Appendages. The antennules are usually short, and in the 

 Oniscoidea they are almost or quite vestigial. They are never 

 biramous except in Bathynomus, where a minute vestige of the 

 inner flagellum is present, and in the cryptoniscan larvae of some 

 Epicaridea. 



The antennae vary much in length, being several times as long 

 as the body in some Asellota, while in the Epicaridea they are 

 hardly more than vestiges. The peduncle usually consists of five 

 segments, but in the Asellota and in Bathynomus and Cirolana it 

 has six. In the Asellota a minute movably articulated scale repre- 

 senting the exopodite is sometimes present on the third segment. 

 In all other Isopoda the exopodite is absent. In the Arcturidae the 

 antennae are very large and strong and are used in seizing prey. 



The mouth-parts of the Isopoda show great diversity of struc- 

 ture, many modifications being correlated with the parasitic or 

 semi-parasitic habits of certain groups. In the more typical forms 

 (e.g. Asellus] the mandible carries a palp of three segments, and has 

 a serrated incisor process, with a lacinia mobilis, at least on the 

 left side of the body, a row of spines, and a strong molar process. 

 The molar process is movably articulated with the body of the 

 mandible in the Cirolaninae (Fig. 120, A). The maxillula (Fig. 

 120, B) has two endites (corresponding, according to Hansen, to 

 the first and third segments) directed distally and bearing strong 

 setae or spines. There is no trace of a palp such as exists in the 

 Tanaidacea. The maxilla (Fig. 120, C) has three endites, the 

 proximal one directly continuous with the basal part and directed 

 obliquely inwards ; the two others are directed distally, over- 

 lapping each other and movably articulated with the basal part ; 

 Hansen regards these as resulting from the division of a single 

 endite corresponding to the third segment. 



The first thoracic limb is always specialised as a maxilliped 

 (Fig. 120, D) and closely associated with the other mouth-parts. 

 It has a short but distinct coxopodite bearing externally a lamellar 

 epipodite (ep) generally more or less indurated, and not, apparently, 

 branchial in function as it is in the Tanaidacea, but serving as an 



