310 PHYLUM ARTHROPODA. 



into an oval mass. Cancer (edible crab) ; Cardnus manas (shore- 

 crab) ; Portunus (swimming-crab) ; Dromia (often covered by a 

 sponge) ; Pinnotheres (living inside bivalves) ; 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 Anaspides 

 and 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 



FIG. 164. Mysisjlexuos&i from side. 



., Brood-pouch borne on posterior thoracic limbs ; <?., otocyst 

 in tail. Note eight rjairs of similar biramose thoracic feet. 

 The last two thoracic segments are not covered by the 

 shield. 



characteristics; but it is difficult to say anything definite as to the 

 more remote ancestry. 



We naturally think of a segmented worm-type as a plausible starting- 

 point for Crustaceans, and it is not difficult to imagine how a develop- 

 ment of cuticular chitin would tend to produce a flexibly jointed limb 

 out of an unjointed parapodium ; how the mouth might be shunted a 

 little backwards, and two appendages and ganglia a little forwards ; 

 and how division of labour would result in the differentiation 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 



