114 
“ TERRA NOVA ” EXPEDITION. 
two thoracic segments, and is not covered by the 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 Eupagurus, where Boas* 
describes the transverse strip as part of the first abdominal tergite. That, however, it 
is not, either in Eupagurus or in Porcellmupagurus. It can hardly be a persistent 
thoracic tergite, since it is not found in lower Deeapoda, 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- 
pagurus the form of a moderately broad transverse plate, lacking the median backward 
expansion which is found in Eupaguru,s. 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 Eupagurusj 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 tergit.es, 
save a vestige on the first segment. But, although calcified remnants of the terga are 
* Loc. cit ., p. 112. 
f The shapes and sizes of the hard pieces of the abdomen vary a good deal from species to species in 
Eupagurus. 
