Page:VCH Herefordshire 1.djvu/167

 CRUSTACEANS digestive cavity. They are succeeded by the first and second maxillae, commonly of slender construction, and furnished usually with a variety of spines and setae, some of which no doubt take a kindly interest in the animal's food. It is difficult to understand how these creatures could deter- mine which part of the world to devour, without some special means of adding zest to appetite by a discrimination of flavours. An observer who starts with the conviction that nature makes nothing in vain will find a fascinating employment in noting the minuter accessories of a crustacean organism. To determine the purposes of all the glassy teeth and hooks, cleft spines and serrate needles, cylindrical filaments and tapering hairs, rigid bristles and softly waving plumes, is a task that requires great nicety of observation and no small ingenuity of logical inference. Some of these purposes, no doubt, are tolerably obvious. Many have been already ascer- tained. But there is always the chance of some new discovery or improved interpretation, and again and again the embarrassing problem recurs, to explain why it is that one species can dispense with some piece of apparatus which is the prominent furniture of another. The maxillae are followed by the first maxillipeds, appertaining to the seventh body segment. At this point it will be convenient to diverge for the moment from the podophthalmous decapod Potamobius, or, in other words, the stalk-eyed ten-footed crayfish, in order to introduce to the reader the edriophthalmous tetradecapod Malacostraca of Herefordshire. These sessile- eyed fourteen-footed orders are not known to comprise, within our limits, more than a single species of Amphipoda, or more than a single aquatic species of the Isopoda. More than one, however, of the terrestrial isopods have been recently added to the fauna. In these groups the eyes, when present, being seated immovably beneath the general integument of the head, show no trace or sign of belonging to an independent ophthalmic segment. Otherwise amphipods and isopods like the crayfish have, in successive pairs, first and second antennae, mandibles, first and second maxillae, and maxilli- peds. Then a striking diffisrence occurs. In the crayfish the eighth and ninth segments of the body carry respectively the second and third maxilli- peds, organs of the mouth, and to the succeeding five segments are allotted five pairs of legs. Above all this apparatus, from the eyes backward, is extended a great shield called the carapace, the cheeks of which enclose the respiratory organs attached to the bases of maxillipeds and the succeeding limbs. When this arrangement is compared with the anatomy of the sessile- eyed groups, though the considerable distinctions at first seize the attention, it is in the end the underlying agreement that forms the chief point of interest. Among the Amphipods the family Gammaridae may be considered typical. Among the Gammaridae Gammarus is the typical genus. In that genus G. pulex (Linn.) is separated from the type species only by small distinctions of form, and the biological difference that it lives in fresh water instead of salt. That it is found in Herefordshire is nothing to boast of, since there is certainly no county in which it is not found. It is scarcely necessary to specify that it has been taken from the River Teme, because it would probably be difficult to find a river or trickling rivulet or pond from which it could not be taken. There is, therefore, little or no difficulty in obtaining "5