CRUSTACEA. 



133 



weed, in cracks of rock, or in pools near the line of low water. 

 Some of them, however, can swim with tolerable activity, and 

 some of them (the land-crabs) even live habitually inland. 

 One group, the " pea-crabs," is distinguished by the singular 

 habit of living semi-parasitically within the shells of bivalve 

 mollusks, such as the great horse-mussel. 



The young or larval crab is exceedingly unlike the adult, 

 and has a long and well-developed abdomen, thus approximat- 

 ing to the type of structure which is permanently retained in 

 the Macrura. 



ORDER ISOPODA (Gr. isos, equal; podes, feet). In this 

 order are a number of Crus- 

 taceans of which some in- 

 habit the sea, others are 

 parasitic in their habits, and 

 others are terrestrial. The 

 best known are the common 

 wood-lice (Oniscus^ Fig. 52), 

 which are found so com- 

 monly under stones, or in 

 the crevices of old walls. 

 The Isopods all belong to 

 a group of Crustaceans in 

 which the eyes are not sup- 

 ported upon stalks, and they 

 are therefore said to be "ses- 

 sile-eyed." The head is dis- 

 tinct from the segment bear- 

 ing the first pair of feet. The 

 thoracic feet are all similar 

 to one another, and the bran- 

 chiae are developed on the 

 abdominal legs. 



FIG. 52. Isopoda. Wood-lice (Oniscus). 



ORDER MEROSTOMATA. In this order are only the living 

 king-crabs (Limulus), and some large extinct forms nearly 

 allied to them. They are all distinguished by the fact that 

 the appendages which are placed round the mouth act by 

 their bases as jaws, but have their extremities developed into 

 swimming-paddles, walking-feet, or nipping-claws. 



The King-crabs or Horseshoe crabs (Fig. 53) constitute a 

 special group called Xiphosura (Gr. xiphos, a sword ; and 

 7 



