Page:VCH Herefordshire 1.djvu/168

 A HISTORY OF HEREFORDSHIRE specimens. The difficulty with the inexperienced will rather be to imagine or believe that this little sliddering creature, about half an inch long, is a poor relation, but a relation not so very distant, of the lordly lobster, and a fit object of comparison with its fresh-water neighbour, the river crayfish. Nevertheless, in all three the integument has the same chitinous foundation, the compound eyes are essentially similar, the number of segments beyond the ophthalmic frontal segment is identical, and the pairs of appendages throughout are strictly comparable. But in Gammarus the carapace stops short at the seventh segment, so that the seven succeeding segments are freely articu- lated. The first two of the seven carry appendages known respectively as the first and second gnathopods, corresponding with the second and third maxilli- peds of the crayfish. Maxilliped, a word of Latin origin, and gnathopod, derived from the Greek, have precisely the same meaning, each signifying jaw-foot. Those who invented these terms wished to intimate that the appendages in question were leg-like structures modified to serve the mouth. The reason for using two terms, where one would have been enough, was this. The maxillipeds are all closely applied to the mouth, and formerly were not thought of as fulfilling other purposes than that of deahng with the food. But the gnathopods are in a more independent position than that which the maxillipeds commonly enjoy. While concerned as a rule in grasping prey and conveying portions of it to the mouth, they are evidently open to other employments. Normally in the Amphipoda they have some claw- like modification, expressed by the term subchelate^ and occasionally they form regular chelipeds, more or less similar to those seen in the first three pairs of legs of the lobster and crayfish. Our own arms are no doubt leg-like struc- tures modified to serve the mouth, but we refrain from calling them either maxillipeds or gnathopods, because they serve so many other purposes in addition to acting as jaw-feet. The five pairs of peraeopods or walking-legs in Gammarus pulex can easily be homologized with those of Potamobius pallipes, but none of them are chelate, and in spite of their name and their simple structure they do not enable the animal to walk on dry land. Like most but not all of their tribe, our fresh-water amphipods, owing to the lateral compression of their bodies, when out of water fall over on one side. Then, by ventral folding and quick unfolding of the tail part, they are able to make a sliding jerking progress not very conducive to speed or comfort. As in the crayfish, the tail part or pleon shows clearly the constituent seven segments, but here the terminal segment known as the telson is longitudinally divided instead of forming a single plate, as in Potamobius. The appendages also differ. In Gammarus the first three segments of the pleon carry pleopods, which, in accordance with their name, are true swimming-legs, while the following three segments carry uropods, literally tail-legs, which assist the animal in its jerking movements. Only the last of the six pairs are called uropods in the crayfish, and these with the telson form the powerful ' tail- fan,' by the flapping of which the animal can propel itself energetically through the water. The five pairs of appendages preceding the tail-fan are here all technically called pleopods, but their swimming power is degenerate, and the first two pairs in the male have been modified into sexual organs. Asellus aquatkus (Linn.) is the only aquatic isopod found, or according to our present knowledge likely to be found, in this county. It shares with ii6