ANNULOSA: CRUSTACEA. 



305 



many are aquatic in their habits, and are often parasitic, but 

 others are terrestrial. 



By Milne-Edwards the Isopoda are divided into three sections, termed 

 respectively, from their habits, the Natatorial, Sedentary, and Cursorial 

 Isopods. In the Natatorial Isopoda the extremity of the abdomen and the 

 last pair of abdominal legs are expanded so as to form a swimming-tail. 

 Some of this section are parasitic upon various fishes (Cymothod), whilst 

 others are found in the sea (Sphceromd). In the Sedentary Isopoda the 

 animals are all parasitic, with 

 short, incurved, hooked feet. 

 This section includes the single 

 family of the Bopyridce, all the 

 species of which live parasitically 

 either in the gill- chambers, or 

 attached to the ventral surface, 

 of certain of the Decapod 

 Crustacea, such as the Shrimps, 

 (Crangones] and the Palamones, 



The Ciirsorial, or running 

 Isopods mostly live upon the 

 land, and are therefore destitute 

 of swimming-feet. The most fa- 

 miliar examples of this section 

 are the common Wood-lice 

 (Oniscus). Here, also, belongs 

 the little Linmoria 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 (Idothed), the 

 Shield-slaters (Cassidina), 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, also, are highly vascular, and respiration 

 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 move- 

 ment." In the Bopyrida the adult females are usually blind, the antennoe 

 are rudimentary, and the abdominal appendages from natatory become 

 respiratory organs. The males, on the. other hand, are dwarfed, and 

 sometimes lose all the abdominal appendages and all traces of segmenta- 

 tion ; 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 Crustacea. 



U 



Fig. 155. Isopoda. Woodlice (Oniscus), twice 

 the natural size. 



