CRUSTACEANS tracans. The latter are so insignificant in size compared with the crayfish, and differ from it so much in general appearance as well as in some obvious details of structure, that an unin- structed observer would be little likely to suspect their near relationship. To Gammarus pulex (de Geer), so widely distributed and so abundant in our brooks and ponds, both Garner and Brown give the vernacular name of freshwater shrimp. Adam White, on the other hand, in his Popular History of British Crustacea, calls it the ' freshwater screw.' 13 In his general survey he had other uses for the term ' shrimp,' which precluded his applying it to any sessile- eyed species. The shrimp or shrimps of commerce, some of which can live in fresh water are Macrura decapoda like the crayfish. But G. pulex, besides having no ocular peduncles, has seven pairs of leg-like appendages, beginning with the eighth instead of the tenth body-segment. Nevertheless these striking differences do not outweigh its other shrimp-like affinities. The eyes, it is true, being seated in the head, give no direct evidence of the initial segment, but the second and third segments in front of the mouth are attested by the two pairs of antennae, a true crustacean characteristic, while at and behind the mouth we find in true malacostracan sequence the mandibles, two pairs of maxillae, and one pair of maxillipeds. The difference which then presents itself is far less schismatical than might at first be supposed. In the higher groups the eighth and ninth pairs of appendages are definitely organs of the mouth, known as second and third maxillipeds. These pairs in the lower groups are concerned more in grasping the food than in mincing it up. They are called gnathopods, a name which can- not well be distinguished by interpretation from maxillipeds, the implication being in each case that the appendages in question are either legs that have made themselves useful as jaws or jaws that have made themselves useful as legs. In the family Gammaridae, of which G. pulex is an excellent representative, the nearly related genera Niphargus and Crangonyx contain species which from their habitat have received the common designation of well- shrimps. It remains to be seen whether the wells of Staffordshire will, like those of some neighbouring counties, yield any of these exceptionally interesting and rather rarely-seen, forms. Like the Amphipoda just described the Isopoda are sessile-eyed. They have, too, the same disposition of the mouth-organs, followed by the legs in seven pairs. In both orders alike the cephalothoracic shield or carapace is only produced to cover the maxillipeds, not as in the Brachyura and Macrura extended to the fourteenth segment of the body. A rather start- ling difference, however, sets the two orders somewhat widely apart. For, whereas the breathing organs of the Amphipoda are, like those of the crayfish, all in front of the pleon, all those of the genuine Isopoda are within it. To counterbalance such separative distinctions among the malacostracan orders, it may be noticed as a unifying character that all along the line the sexual openings of the female belong to the twelfth body-segment, and those of the male to the fourteenth. Of freshwater isopods our Mediterranean counties, as Plot calls them, have only one species, the proper name of which is, not Asellus vulgaris (Latreille), but Asellus aquaticus (Linn.). It has as much or as little right as Gammarus pulex to be called the fresh- water shrimp. To call it, as Brown does, the freshwater Asellus, is not much to the purpose, because in this genus, established by Geoffroy in 1762, all the species belong exclusively to fresh water. It may also be thought superfluous to have the typical species named aquaticus, since none of the species are other than aquatic. But the explanation is found when we look a little further back into its scientific history. Linnaeus regarded it as belonging to the old comprehensive genus Oniscus, which at one time included all the terrestrial isopods, so that a species found constantly in water and nowhere else could naturally be distinguished as a water- dwelling Oniscus. Again, among the land-dwelling species Oniscus asellus, Linn., was the most familiar, so that Geoffroy, when separating the aquatic species from its sub-aerial companions, may have thought it well to preserve a memory of the old connexion by taking Asellus as the name of his new genus. The differences between the two species which are thus partially namesakes are now recognized as very considerable, with the result that Asellus aquaticus is allotted to a family Asellidae in the tribe Asellota, while Oniscus asellus stands in a family Oniscidae in the tribe Oniscidea. Concerning the large light-coloured form to which Mr. Brown alludes as possibly deserving to be specifically distinguished from the last-named species, the caution may be expressed that in some of our common land isopods variations of colour appear without affecting their other characteristics. This is eminently true of the next species, Porcellio scaber, Latreille. It belongs to the same family as the Oniscus, is nearly its equal in size, and perhaps fully its equal in abundance. It is rather narrower in shape and "Op. cit. (1857), p. 184. I 129 17
