114 "TERRA NOVA" EXPEDITION. 



two thoracic segments, and is not covered by tlie sternum of the fourth pair of legs 

 because the latter has receded to a more dorsal position than that which it usually 

 occupies. 



The condition of the abdomen in the living animal has, unfortunately, not been 

 described. In spirit specimens (Figs. 2, 5, 13a) it forms a rounded sack, placed behind 

 the cephalothorax. From the last thoracic segment it is separated by a groove, fairly 

 deep on the ventral side, but little marked above. In front of that segment, however, 

 there is a greater furrow, by which, as by a waist, the body is divided into two regions, 

 one consisting of the major part of the cephalothorax, the other of the abdomen 

 together with the last thoracic segment. The waist also is deepest on the ventral 

 side. The abdomen is a good deal flattened above but bellies below. It is possible, 

 though perhaps not likely, that its length is greater in living than in preserved 

 specimens, in which case the true aspect of the animal might be considerably less 

 crab-like than that under which it is at present known. 



Where the thorax joins the abdomen there lies across the back a narrow transverse 

 strip of hard cuticle (Fig. 13a), which has at least the appearance of being the tergite of 

 the last thoracic somite. Its ends abut on a pair of oval plates of like substance, placed 

 one above the base of each of the legs of the segment, and perhaps to be regarded as 

 pleural structures. A similar arrangement is found in Eiipagurus, where Boas* 

 describes the transverse strip as part of the first abdominal tergite. That, however, it 

 is not, either in JEupagurus or in PorcellanoiMgurus. It can hardly be a persistent 

 thoracic tergite, since it is not found in lower Decapoda, and may perhaps be more 

 correctly described as a structure sui generis than as a tergite at all ; but in both genera 

 it lies clearly in the thoracic region, and can be distinguished from the first abdominal 

 tergite, which lies behind it, and from which is formed the opposite face of the thoraco- 

 abdominal groove, along whose floor in Eupagurus there runs a fine, white, transverse 

 line like a suture. The two tergal sclerites are, however, firmly united, and together 

 provide a necessary strengthening of the back in the region of the attachment of the 

 last pair of legs. The true tergite of the first abdominal segment has in Porcellano- 

 p>agurus the form of a moderately broad transverse plate, lacking the median backward 

 expansion which is found in Eupagurus. A pair of independent plates, of which the 

 left bears a limb, stand in the female for the second tergite ; a smaller plate bearing a 

 limb is the remains of the third tergite, while at the base of the limb of the fourth 

 segment there is barely a trace of such a thickening. The fifth segment is altogether 

 soft. This arrangement is derived from that of Eupagurusf by the disappearance of 

 the plate on the right hand side of the third and fourth segments, and of the whole 

 tergite of the fifth. In the male (of P. tridentatus) there are no abdominal tergites, 

 save a vestige on the first segment. But, although calcified remnants of the terga are 



* Loc. cit., p. 112. 



f The shcapes and sizes of the hard pieces of the abdomen vary a good deal from species to species in 

 Eupagurus. 



