ANNULOSA : CRUSTACEA. 245 



Wood-lice (Oniscits). Here, also, belongs the little Limnoria 

 terebrans, so well known for the destruction which it produces by 

 boring into the wood-work of piers and other structures placed 

 in the sea. Other well-known Isopods are the Water-slaters 

 (Asellus} of fresh waters, the Rock-slaters (Ligia) of almost all 

 coasts, the Box-slaters (Idothea), the Shield-slaters (Cassidind), 

 and the Cheliferous Slaters (Tanais). These last are remarkable 

 as being the only Isopods in which there is a carapace. The 

 lateral parts of the carapace are highly vascular, and respira- 

 tion is effected by these, and not by the abdominal feet. 



Many Isopods undergo an extensive metamorphosis. " In 

 some Fish-lice ( Cymothoa) the young are lively swimmers, and 

 the adults are stiff, heavy, stupid fellows, whose short clinging 

 feet are capable of little movement." In the Bopyridce the 

 adult females are usually blind, the antennae are rudimentary, 

 and the abdominal appendages from natatory become respira- 

 tory organs. The males, on the other hand, are dwarfed, 

 and sometimes lose all the abdominal appendages, and all 

 traces of segmentation \ until we get forms which, like 

 Cryptoniscus planarioides, " would be regarded as a Flat-worm 

 rather than an Isopod, if its eggs and young did not betray 

 its Crustacean nature " (Fritz Miiller). 



DIVISION B. PODOPHTHALMATA. The members of this divi- 

 sion have compound eyes supported upon movable stalks or 

 peduncles, and the body is always protected by a cephalo- 

 thoracic carapace. Most of the Podophthalma pass through 

 Zoea-stages in their development. It comprises the two orders 

 Stomapoda and Decapoda, of which the latter includes all 

 the highest and most familiar examples of the class Crus- 

 tacea. 



ORDER I. STOMAPODA. In this order there are generally 

 from six to eight pairs of legs, and the branchiae, when present, 

 are not enclosed in a cavity beneath the thorax, but are either 

 suspended beneath the abdomen, or, more rarely, are attached 

 to the thoracic legs. The shell, also, is thin, and often mem- 

 branous. From all the preceding orders the Stomapoda are, of 

 course, distinguished by the possession of pedunculate eyes. 

 The development of the Stomapoda would appear to be by 

 means of "Zoeae." 



All the Stomapods are marine, with the single exception 

 of the My sis relicta of the great lakes of Sweden and North 

 America ; and the Locust Shrimp (Squilla mantis] may be 

 taken as a good example of the order. In this Crustacean the 

 carapace is small, and the posterior half of the thorax is unpro- 

 tected. Several of the anterior appendages are developed into 



