i8o THE FLOATING ANIMALS 



Doliolum (see p. 186). Very closely allied to these is 

 the subdivision which includes the common wood-lice 

 or pill-bugs ; these are not insects, as is generally 

 thought, but belong to the subdivision Isopoda ; they 

 may generally be distinguished from the previous sub- 

 division by being flattened from above downwards, 

 while the former are flattened from side to side. Few 

 of them occur in Plankton on the high seas, except as 

 parasites, either temporary or permanent (Fig. 116). 

 Rather unexpectedly, considering their size and weight, 

 some of the Stomatopoda may be found swimming 

 near shore (Fig. 119) ; they should not be handled, as 

 their big claws, shutting like a pocket-knife, can cut 

 a finger to the bone. 



The " opossum-shrimps " (Schizopoda), so called 

 from the brood-pouch present in some species, are fre- 

 quently taken. Several have special luminous organs, 

 provided with lens and reflector, in different parts of 

 their bodies (Fig. 118). The subdivision, which includes 

 the lobster and shrimp, is termed Macrura the 

 " long-tails "; few of these are planktonic when adult, 

 but some prawn-like forms are widely found. Mention 

 must be made, however, of the curious little Sergestids 

 (Fig. 130), not only because they are often captured, 

 but also because of their curious larvae (Fig. 129). Less 

 adapted to a swimming life are the Brachyura, or 

 " short-tails," of which the common crab is a member ; 

 some can swim a little, and one at least has been found 

 in quantity about 200 miles from shore over very deep 

 water. Crabs are also found attached to gulf-weed 

 and other floating objects. 



The larvae of Crustacea are often even more numerous 

 than the adults. Most marine Crustacea hatch out of the 

 egg in the form called a Nauplius, which may be known 

 by a shield-shaped body, a single eye, and three pairs. 



