310 PHYLUM ARTHROPODA. 
into. an oval mass. Cancer (edible crab); Carcénus manas (shore- 
crab); Portunus (swimming-crab); Dromia (often covered by a 
sponge).; Pinnotheres (living inside bivalves); Ze/phusa (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 Zs¢herda, living from 
Devonian ages till now, are remarkably persistent and successful. How 
the class arose we do not know ; itis probable that types like Anaspzdes 
and Nebalza give us trustworthy hints as to the ancestors of the higher 
Crustaceans ; it is likely that the Phyllopods, e.g. Afus, bear a similar 
relation to the whole series ; the Copepods also retain some primitive 
ELK: 
Niet 
Fic. 164.—Mysis flexuosa, from side. 
&., Brood-pouch borne on posterior thoracic limbs ; 0., otocyst 
in tail. Note eight pairs of similar biramose thoracic feet. 
a 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 
