Page:VCH Bedfordshire 1.djvu/134

 A HISTORY OF BEDFORDSHIRE were distinct one from another, but he would not easily persuade the latter two to live in fresh water, in which the first finds itself happy and at home. Without going into every detail, it may be pointed out that the sides of the head project angularly in G. locusta, but are rotund in G. pu/ex, and that the accessory branch of the upper antenna? is more numerously jointed and longer in the former than in the latter. At the other extremity of the organism the last appendages, known as the third uropods, pretty well agree in these two species by having two elongate branches ; but in G. marinus the inner branch, instead of being only a little shorter than the outer, is scarcely a third of its length. In the Isopoda the appendages of the pleon form five pairs of pleopods (swim- ming feet) and one pair of uropods (tail feet). In the gammaridean Amphipoda there are three pairs of pleopods and three of uropods. In these pleopods each appendage consists of a stem with two many-jointed flexible setose branches, and this description, though admitting of some exceptions, applies with extraordinary uniformity throughout the group. When the animal is alive in its own element its natatory limbs are in tolerably constant motion, and that even when the body is stationary. The purpose of this is to keep a current of water continually bathing the branchial sacs, and when there are eggs in the mother's pouch to give them also the benefit of the invigorating stream. The maternal marsupium is formed by membranaceous lamina? which, like the respira- tory vesicles, are attached to several of the trunk-legs at the upper part. In contrast to the pleopods the uropods have branches which are not flexible. They seem to be purely locomotive, enabling the animal to make leaps or to jerk itself along, or in tube-building species to execute manoeuvres which may best be described as turning head over heels. In almost every marine province the Malacostraca form a demon- strative portion of the fauna. It is only in specially favoured regions that they are conspicuous as occupants of fresh water. The other half of the crustacean class, upon which O. F. Miiller, towards the close of the eighteenth century, bestowed the designation Entomostraca, is rather differently situated. For, though these so-called ' insects covered by a shell ' are extremely abundant in the sea, they also inhabit inland waters, not only with surprising diversity of form and ubiquity of range, but with no sort of regard for Malthusian prudence in their rate of prolifera- tion. In regard to this division of the subject there is fortunately some- thing to be gleaned from existing records. One quotation indeed might have been earlier given as applying not to a part of the class, but to the whole. The statement is of a character rather elementary, yet with certain reserves useful to be borne in mind. In a paper on ' Antennas,' Mr. H. J. Sheppard begins by saying : 'Antenna? are horn-like members placed on the head, peculiar to insects and crustaceans ; the former generally have two, the latter more than two.' ' To this it might be objected that horns are usually stiff and antenna? usually flexible ; and 1 Abs. Proc. Trans. Beds Nat. Hist. Soc. and Field Club for the years 1877-81 (Oct. I, 1882), p. io. 94