Page:VCH Herefordshire 1.djvu/166

A HISTORY OF HEREFORDSHIRE Without further lingering on more or less conjectural or inferential members of the fauna, we may return to the consideration of those which have direct testimony guaranteeing their occurrence in this county. Though the number of observed species is still very small, both the principal divisions of the crustacean class, the Makcostraca and Entomostraca, are moderately represented. It is true that of stalk-eyed malacostracans there is only a single species. But, so far as that is concerned, none of our strictly inland counties are any better off. Moreover there is this consolation, that the crayfish is itself an admirable example of the class for the purposes of the student. It is easily procurable and tenacious of life, so that its habits in captivity can be observed at leisure. As a specimen it is convenient for handling and dissection, the parts being neither awkwardly large nor microscopically small. With a view to comparative anatomy the want of nearly allied species can be easily remedied. Crab and lobster, prawn and shrimp, though not living within these borders, can without difficulty be obtained from elsewhere. Not only the lobster, but also the other apparently very different forms, when carefully examined will show a long series of agreements in their succession of articulated appendages and in the other structures by which their vital activities are maintained. It would be out of place here to discriminate the genera included under these popular and familiar names, but it is desirable to note some of the many features which they have in common. They are all stalk-eyed — that is, their paired organs of vision are seated on movably articulated peduncles. In some crustaceans, though not in any of the common eatable species just mentioned, the eye-stalks attain a considerable elongation. This and some other circumstances make it probable that the eye-stalks of the decapod Makcostraca are comparable in character and origin with the remaining pairs of appendages, nineteen in number, which these organisms have at their disposal. Theoretically each pair belongs to a separate body segment, and the theory is undoubtedly sound, although there is no species known in which the separateness of the segments is clearly maintained from one end of the body to the other. Also there is a twenty-first segment, called the telson, which is perhaps potentially appendage-bearing, although it never has a distinctly separated pair of appendages. No makcostracan, be it observed, has more than a score of such pairs, and oftentimes some of the score are missing. The complete number is small compared with that occurring in some of the centipedes, but in the latter group the manifold repetition is for the most part of limbs alike in appearance and function. Here, on the contrary, adaptation to a great variety of purposes and conditions has given rise to extreme diversity of form. The normal makcostracan has two pairs of antennae, belonging respec- tively to the second and third segments of the body. Associated with the first of these pairs is the sense of hearing. The seat of the olfactory sense is not quite so definitely determined. The sense of touch is, no doubt, more or less generally diffused. The sense of taste, if disjoined from that of smell, may not unnaturally be sought among the more delicate of the mouth-organs. The mandibles, belonging to the fourth segment, are often massive, with cut- ting or piercing capacity, but their stronger processes are frequently attended by slender spines and setae which must have some meaning. The mandibles meet between the upper and lower lips, which form the entrance to the 114