Introduction to Animal Morphology. 357 



Gasteropods. Coenobita is a terrestrial form, which, in the 

 West Indies, uses the shells of Bulimi. Pagurus is the 

 common hermit crab. Birgus latro, the tree crab, is said to 

 break open cocoa nuts. 



Sub-order 3. Brachyura {Latreille) crabs — post-abdomen 

 short, compressed, turned up in a hollow of the sternum under 

 the cephalothorax, with no swimmeret on the last joint, and 

 thread-like pleiopods ; antennae short ; the dorsal surface is 

 marked medially from before backward into gastric, genital, car- 

 diac and intestinal regions, laterally into hepatic and branchial : 

 the foremost pereiopod is chelate ; the male post-abdomen 

 is narrow, lanceolate ; the female broadly ovate ; between it 

 and the sternum the eggs are carried in clusters ; the fore- ■ 

 most of the two rudimental pleiopods in the males is a clasper ; 

 in the female the same acts as a support organ for the gills, 

 which are 7-9 on each side ; the nervous system is concen- 

 trated into a single peripharyngeal ring. The larva or Zoea 

 is characterised by having a cephalothoracic shield with a 

 central spine, and a long post-abdomen without appendages. 



The following families are included: — i. Catometopa — 

 cephalothorax four-angled ; gills fewer than nine ; male 

 sexual orifice sternal. Ocypode has a flagellum from the 

 ommatophore, continued past the eye, supposed to indicate 

 the relation of the stalk to a limb. Pinnotheres lives be- 

 tween the valves of molluscs. Thelphusa is a S. European 

 and N. African river crab. Gecarcinus, a W. Indian land 

 crab, has between its gill lamellse hard processes, which pre- 

 vent them closing, and allow in air. 2. Cyclometopa — cara- 

 pace oval, transversely elongate ; gills nine ; male sex-organs 

 in the hip-joint of the fifth pereiopod. Cancer, the edible 

 crab, has a serrated anterior edge. Portunus is rounded in 

 front, and toothed laterally ; the fifth pereiopod is natatory. 

 Corystes is antero-posteriorly elongate. Carcinus has long 

 postero-lateral edges to its carapace. 3. Oxyhryncha, spider 

 crabs — carapace trigonal, pointed in front, an anterior pro- 

 boscis. 4. Dromiidae — fifth pereiopod smaller than the others, 

 displaced dorsally ; gills fourteen. Dromia is covered with 

 brown hair. 5. Oxystomata — mouth triangular, not quadrate, 

 as in the foregoing, with a rostrum in front ; gills often only 



