Page:VCH Warwickshire 1.djvu/215

CRUSTACEANS two-branched, each branch setiferous, consisting of only 2-4 joints. Mandibles quite devoid of palp. Pairs of feet 46, of which for the most part the majority or all are foliaceous, lobed. The eye single.' 1 Freedom is a word of many meanings. Minuteness is a matter of comparison. The objects of the above definition are free in contrast to many entomostracans which are parasitically attached to other organisms. In size they range, with a few exceptions, between about one-fifth and one-hundredth of an inch, so that there are some living creatures indefi- nitely smaller than the smallest of them. The distinctness of the head is noted to contrast them with the Ostracoda, which have the head as well as the rest of the body enclosed in a bivalved test or shell covering. Their lateral compression is a character not uncommon, but in the Branchiura, in many Phyllopoda and in the Copepoda as a rule the compression is dorso-ventral, from above downwards. The branching second antennas are so characteristic that the name of the whole sec- tion alludes to this feature, and though the joints in each branch are so few, the varying numbers admit of many combinations useful in distin- guishing genera. In the absence of a palp from the mandibles nature here speaks with unwonted decision. Elsewhere we find crustacean groups in which some members have this palp and others are without it. Such a difference between nearly related genera or species seems very capricious, as though it were introduced just to try the temper of systematists. The mandible may be regarded as an appendage originally similar to the many-jointed limbs. Its basal part became enlarged and fortified for purposes of mastication, and the slender terminal joints, now spoken of as 'the palp,' have in some cases entirely disappeared, in others been partially retained. This may be explained, in the terms of modern science, as an example of the continual struggle between heredity on the one hand and adaptation to circumstances on the other. The Cladocera are divided into two principal companies : the Calyp- tomera, a name implying that the limbs are covered by a well developed carapace ; and the Gymnomera, or bare shanks, in which the carapace is small and does not encompass the trunk limbs. Each company is sub- divided into two tribes. The first tribe of the Calyptomera takes its descriptive name, Cten6- poda, or comb-feet, from the fact that all its six pairs of foliaceous legs are furnished with setae arranged like the teeth of a comb. One of its families, the Sididae, contains two genera recorded for this county Sida, Straus, and Diaphanosoma, Fischer. In the former the dorsal, outer, or upper branch of the second antennae has three joints, and the ventral, inner, or lower branch only two ; while the reverse is the case in the latter genus. The species Sida crystallina (O. F. Miiller) is stated by Mr. Hodgson to be ' abundant in clear weedy pools and canals.' It has on the back of its head an apparatus by which it can affix itself to one or other of the aquatic plants among which it dwells. It is also distin- guished by having the dorsal margin of its post-abdomen fringed with 1 Annaks Jei Sciences Nature/let, ser. 7, xviii. 304 (1895). I 177 23