A STUDY IN CARCINOLOGY. 35 
moves the coxa oiiiwardsj backwards, and slightly upwards ; the innermost 
and slightly more ventral muscle-cavity encloses the adductor muscles whose 
notion is the reverse of the abductors. 
On comparing the thoracic skeleton of a crab such as Carcimis manias 
with that of a crayfish or a lobster, the differences are considerable. The 
■carapace being removed, the epimera are seen to slope inwards towards the 
median dorsal line like the roof o£ a house. The sterna instead of being- 
narrow and keel-like are widely extended laterally, and form by their union 
tlie broad ventral plastron across which run a series of transverse grooves 
indicating the sternal apodemes. As a consequence the sockets of the limbs 
are placed not ventrally but on the sides of the body, forming a row which is 
not straight but curved upwards at both ends. In some cases the hinder end 
of the row curves upwards, the fourth pereiopods being dorsal in position. 
Although Dromia is in many respects a primitive crab retaining many 
Macruran features, its endophraginal skeleton is so characteristically crab- 
like that it will serve for comparison and contrast with that of the Crayfish. 
Pi. 4. fig. 12 is a drawing of the left side of the thorax of Dromia vulgaris 
after removal of the carapace. The thorax lias been slightly tilted over to 
the right to bring it as nearly as possible into comparison with PI. 4. fig. 11, 
and for simplicity's sake the perspective of the ventral surface of the sternum 
has been omitted. Fig. 15 (PI. 5) is a posterior view of the same preparation 
which illustrates some I'eatures not clearly shown in fig. 12. 
As in Asftacus, the stropliingia and stropliidia, the former situated at the 
upper ends the latter at the lower ends of the arthrophragms, serve as 
convenient landmarks. The most conspicuous feature in the crab is the 
great depth of the arthrophragms, especially those between the muscle- 
■cavities of the segments of the second and third and third and fourth 
pereiopods (intersegments xii/xiii and xiii/xiv). It is also characteristic 
that- the three posterior arthrophragms slope forward, tiie two anterior 
■arthrophragms backward, in such wise that the flexor muscle-cavities of 
which they form the walls converge towards a point situated in the middle 
■of the thorax. A third characteristic feature is that, owing to the low pitch 
of the gabled roof formed by the epimera (see PI. 5. fi^'. 15) the abductor 
•muscle-cavities lie above and not, as in the Macrura, in the same horizontal 
plane as the adductor cavities. All these features, which go a long way to 
make up the characteristic cancroid facies, are correlated with the habits of 
crabs. They run much more actively than any Macrura, and the attach- 
ments for the muscles of their pereiopods are proportionally increased. To 
provide these attachments the intersegmental apodemes are very deep, 
pirticularly on the sternal surface and on the sides of the body, and tlie 
arthrophragms, instead of being mere flattened bars as in the Macrura, are 
■conspicuous partitions extending deep into the body. But their internal 
3* 
