210 



NATURAL HISTORY. 



Fig. 30. OPOSSUM SHRIMP (Mysis cUamaleon). 



From the stalk-eyed Podophthalmia we pass to the sessile-eyed Edriophthalmia, Crustaceans 

 in which (with few exceptions) the eyes are fixed immediately on the surface of the head. As in the 

 higher forms, the eyes are compound, consisting in the young of some ten or twelve lenses only, but 



in the adult of as many as from sixty to eighty. The 

 head-shield in the Crab and Lobster encroaches so far 

 upon the body- segments as frequently to conceal them, 

 whereas in the EDRIOPHTHALMIA the head-shield only 

 covers the seven first, or head rings, the seven thoracic 

 segments being well developed in both divisions and 

 the seven abdominal also in the Amphipoda, but in 

 the Isopoda they are mostly coalesced together. The 

 body in the Amphipods is compressed at the sides, 

 whereas in the Isopods it is mostly broad and flattened in shape. Thus the Isopods and Amphi- 

 pods form two very natural groups, which are comparable to the Crabs and Lobsters, the Isopods 

 resembling the former and the Amphipods the latter in shape. 



SECOND LEGION. EDRIOPHTHALMIA. 



ORDER III. ISOPODA (EQUAL-FOOTED). 



The ISOPODA are so named in allusion to the general conformity in size and function of the 

 seven pairs of legs, the two foremost pairs of which in the AMPHIPODA are equivalent to the 

 two outer pairs of jaw-feet in higher Crustacea. 



In the DECAPODA, STOMAPODA, and AMPHIPODA, the branchiae (in each case) are attached 

 to the base of the legs. In the Isopoda, on the contrary, the posterior (abdominal) appendages are 

 converted into special organs of respiration, in the form of leaf-like appendages The heart is also 

 near the tail. The body is composed of seven segments, generally nearly equal in size. To these, 

 in the normal Isopods, seven pairs of nearly uniform legs are attached, either fitted for walking, 

 swimming, or as powerful hook-like organs which enable them to adhere firmly to the fishes upon 

 which many of them are parasitic. 



One group of Isopods, the Oniscidce, familiar to us in gardens under the name of the Common 

 Wood Louse, are all air-breathers not residing in water, but in damp situations breathing air, 

 which, however, it is necessary should be saturated with moisture. Several of the species which 

 inhabit caves are destitute of eyes, e.g., Titanellies albus, from the Mammoth Cave, Kentucky. The 

 Great Sea-Slater (Lygia oceanica) is common on the British coasts, running with agility, and folding 

 up, so as to feign death, when attacked. 



One of these, found in some gardens and woods, the Armadillo (also named the " Pill Bug " in 

 America), from the perfect way in which the segments roll together, forcibly reminds one of the fossil 

 genus Illcenus barriensis, a Trilobite found in the 

 Upper Silurian at Barr, Staffordshire. 



Prof. Alexander Agassiz, aided by the United 

 States Coast Survey, has carried on extensive deep- 

 sea dredging operations in the American seas, particu- 

 larly exploring the bed of the Gulf Stream and the 

 Straits of Florida, between the south point of Florida 

 and the Island of Cuba. Among other Crustacean 

 treasures obtained was a gigantic Isopod, dredged from 

 a depth of 955 fathoms, on the north-east of the bank 

 of Yucatan, and north of Tortugas. This Isopod has 

 been named Bathynomus giyanteus(Fig. 31) byAlphonse 

 Milne-Edwards. It measures nine inches in length by 



four inches in breadth, and far exceeds any other living Isopodous Crustacean in dimensions. The gills 

 or branchiae, which in ordinary Isopods are simple leaf-like appendages, formed out of the modified 

 abdominal feet, in Bathynomus consist of a highly complex arrangement of tufts of filaments 

 Supported on tubular peduncles covered by a series of opercular plates. Notwithstanding the vast 



Fig. 31. GIGANTIC ISOPOD (Bathyiwmus). 

 a, side view ; 6, front view of head. 



