ENTOMOSTRACA. 541 



between the widely-extended stomach and the walls of the body 

 and limbs is occupied by a transparent liquid, in which are seen 

 floating a number of minute transparent corpuscles of irregular 

 size ; and this fluid, which represents the blood, is kept in con- 

 tinual motion, not only by the general movements of the body 

 and limbs, but also by the actions of the digestive apparatus ; 

 since, whenever the csecum of any one of the legs undergoes dila- 

 tation, a part of the circumambient liquid will be pressed out 

 from the cavity of that limb, either into the thorax, or into some 

 other limb whose stomach is contracting. The fluid must obtain 

 its aeration through the general surface of the body, as there are 

 no special organs of respiration. The nervous system consists of 

 a single ganglion in the head (formed by the coalescence of a 

 pair), and of another in the thorax (formed by the coalescence of 

 four pairs), with which the cephalic ganglion is connected in the 

 usual mode, namely, by two nervous cords which diverge from 

 each other to embrace the oesophagus. Of the reproduction of 

 this animal, nothing is yet known. In the study of the very 

 curious phenomena exhibited by the digestive apparatus, as well 

 as of the various points of internal conformation which have been 

 described, the achromatic condenser will be found useful, even 

 with the 1 inch, 2-3ds inch, or \ inch objectives ; for the imper- 

 fect transparence of the bodies of these animals renders it of im- 

 portance to drive a large quantity of light through them, and to 

 give to this light such a quality, as shall define the internal or- 

 gans as sharply as possible. 



366. The Entomostracous group of Crustaceans, nearly all the 

 existing members of which are of such minute size as to be only 

 just visible to the naked eye, is distinguished by the enclosure 

 of the entire body within a horny or shelly casing, which some- 

 times closely resembles a bivalve shell in form and in the mode 

 of junction of its parts, whilst in other instances it is formed of 

 only a single piece, like the hard envelope of certain Rotifera 

 ( 282, III). The segments into which the body is divided, are fre- 

 quently very numerous, and are for the most part similar to each 

 other; but there is a marked difference in regard to the appen- 

 dages which they bear, and to the mode in which these minister 

 to the locomotion of the animals. For in the Lophyropoda^ or 

 bristly-footed tribe, the number of legs is small, not exceeding 

 five pairs, and their function is limited to locomotion, the respi- 

 ratory organs being attached to the parts in the neighborhood of 

 the mouth ; whilst in the Branchiopoda, or gill-footed tribe, the 

 same members serve both for locomotion and for respiration, 

 and the number of these is commonly large, being in Apus not 

 less than sixty pairs. The character of their movements differs 

 accordingly ; for whilst all the members of the first-named tribe 

 dart through the water in a succession of jerks, so as to have 

 acquired the -common name of "water-fleas," those among the 



