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 



