MARINE CRUSTACEANS. 575 



the crab, for the sponge is quite free, and is merely held in position by its host. On the 

 other hand, it must Live thus for some time, and grow meanwhile, for it will always be 

 found to have a deep hollow on the underside, closely fitting the back of the crab. No 

 doubt this helps to keep it from slipping, as does also, perhaps, the hairy or woolly coat 

 with which the decapod is usually provided. The hairs of this coat are often very diverse 

 and complicated in shape — pointed or clubbed, feathered or simple (PL XXXIII, figs. 1 b — d, 

 3c— f^) — though it is hard to see what end these can serve. 



Of the two groups into which the Dromiacea are subdivided, the Homolidea are princi- 

 pally deep-water forms, and were not met with by the expedition. The Dromiidea, on the 

 other hand, are fairly common on the reef where they are to be found in all possible 

 positions, holding on tightly with the sharp tips of their very mobile walking-legs (PI. 

 XXXIII). In their habits they are slow and heavj', trusting rather to concealment than 

 to active avoidance and defence. 



In the systematy of the Dromiidea much use is made of the sternal grooves of the 

 female. These are a pair of furrows, one on each side, which run longitudinally over the 

 thoracic stemites of the female, beginning on the hindermost and ending on that which 

 corresponds to the first, second, or third pair of legs. Their forward ends may be raised 

 on knobs, and are either wide apart, side by side but not meeting, or curving round to 

 meet. They correspond closely \vith the position in which the long abdominal limbs of the 

 male are carried, and are therefore probably used for some sexual purpose. Other features 

 of systematic importance in this group are : the presence or absence of epipodites, the pro- 

 portions of the body and the extent to which furrows are present on the back, dividing 

 it into regions, the shape and relative length of the legs, etc. For specific distinctions, the 

 shape, number and arrangement of the teeth on the front, anterolateral edge, underside of 

 the bod}', and legs are chiefly used. 



The following keys, introduced here in performance of the promise given in the article 

 on the classification of the crabs (p. 426 above), will enable members of the legions and 

 families of the Dromiacea to be recognised : 



Key to (he legions of the Dromiacea. 



I. Sternum of female with longitudinal grooves. Vestiges of 6th pair of abdominal limbs 

 usually present. Gills 14 — -20 on each side. Eyes usually completely sheltered by orbits when re- 

 tracted. No linear anomuricae. Dromiidea. 



II. Sternum of female without longitudinal grooves. No vestiges of 6th pair of abdominal 

 limbs. Gills 8 — 14 on each side. Eyes incompletely or not at all sheltered by orbits when with- 

 drawn against the body. Lineae anomuricae usually present. Homolidea. 



Key to the families of the Dromiidea. 



I. No vestige of 6th abdominal limb. Carapace longer than broad, with ill-marked side edge. 

 [First three legs with epipodites, fourth and fifth small, subdorsal, and prehensile.] Homolodromiidae. 



1 For the investigation of this interesting point I am indebted to Mr Edwin Wilson's enthusiasm for the subject. 



