GENERAL NOTES ON CRUSTACEANS. 279 



shrimp) ; PahEtnon, Pandalus, Hippolyte (prawns) ; Galathea 

 (with the abdomen bent inwards) ; Pagurtis, Etipagurus 

 (hermit crabs) ; Birgus latro (the terrestrial robber or palm 

 crab), in which the upper part of the gill cavity is shut off to 

 form a " lung," the walls having numerous vascular plaits. 

 Opinion seems to incline against recognising a separate sub- 

 order (Anomura) for the soft-tailed hermit crabs. 

 Sub-order 2. Brachyura. Abdomen short, and bent under the 

 thorax. It is narrow in the male, and does not usually bear 

 more than two pairs of appendages ; it is broader in the 

 female, and bears four paired appendages. The ventral 

 ganglia have fused into an oval mass. Cancer (edible crab) ; 

 Carcinus m&nas (shore crab) ; Portunus (swimming crab) ; 

 Maia (spider crab) ; Lithodes (stone crab) ; Porcellana ; 

 Dromia (often covered by a sponge) ; Pinnotheres (living 

 inside bivalves) ; Gelasimus (fiddler crab, a very adept 

 burrower) ; Telphusa (a fresh water crab) ; Gecarcinus (land 

 crabs, only visiting the sea at the breeding season). 

 * History. Fossil Crustaceans are found in Cambrian strata, but the 

 highest forms (Decapoda) were not firmly established till the Tertiary 

 period. Some of the genera, e.g., the Branchiopod Estheria, living 

 from Devonian ages till now, are remarkably persistent and successful. 

 How the class arose, we do not know : it is probable that types like 

 Nebalia give us trustworthy hints as to the ancestors of the higher 

 Crustaceans ; it is likely that the Phyllopods, e.g., Apus, bear a similar 

 relation to the whole series ; the Copepods also retain some primitive 

 characteristics ; but it is difficult apart from mere guessing to say any- 

 thing definite as to the more remote ancestry. 



We naturally think of a segmented worm type as a plaus- 

 ible starting-point for Crustaceans, and it is not difficult 

 to understand how a development of cuticular chitin would 

 tend to produce a flexibly jointed limb out of an unjointed 

 parapodium, how the mouth might be shunted a little back- 

 wards, and two appendages and ganglia a little forwards, 

 and how division of labour would result in the differentia- 

 tion of distinct regions. 



GENERAL NOTES ON CRUSTACEANS. 



Of a class that includes animals so diverse as crabs, 

 lobsters, shrimps, "beach fleas," "wood lice," barnacles, 

 acorn shells, and " water fleas," it is difficult to state general 

 characteristics, other than those facts of structure which we 

 have already summarised. 



Admitting the parasitism of many Crustaceans, and the 

 sedentary life of barnacles and acorn shells, we must still 



