CRUSTACEA. 207 



ful, even with the 1 inch, 2-3d inch, or % inch Objective; for the imper- 

 fect transparence of the bodies of these animals renders it of importance 

 to drive a large quantity of light through them, and to give to this light 

 such a quality as shall sharply define the internal organs. 1 



603. ENTOMOSTRACA. This group of Crustaceans, nearly all the ex- 

 isting members of which are of such minute size as to be only just visible 

 to the naked eye, is distinguished by the inclosure of the entire body 

 within a horny or shelly casing; which sometimes closely resemble a bi- 

 valve 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 ( 453, in.). The segments into which the body is 

 divided, are frequently very numerous, and are for the most part similar 

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

 ages which they bear, and to the mode in which these minister to the lo- 

 comotion 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 respiratory organs being attached 

 to the parts in the neighborhood of the mouth; whilst in the Branchio- 

 poda, or 'gill-footed* tribe, the same members (known as 'fin-feet') 

 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 differ 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 latter 

 which possess a great number of ' fin-feet,' swim with an easy gliding move- 

 ment, sometimes on their back alone (as in the case with Branchipus), and 

 sometimes with equal facility on the back, belly, or sides (as is done by 

 Artemia salina, the 'brine shrimp'). Some of the most common forms 

 of both tribes will now be briefly noticed. 



604. The tribe of Lophyropoda is divided into two Orders; of which 

 the first, Ostracoda, is distinguished by the complete inclosure of the 

 body in a bivalve shell, by the small number of legs, and by the absence 

 of an external ovary. One of the best known examples is the little Cypris, 

 which is a common inhabitant of pools and streams: this may be recog- 

 nized by its possession of two pairs of antennae, the first having numerous 

 joints with a pencil-like tuft of filaments, and projecting forwards from 

 the front of the head, whilst the second has more the shape of legs, and 

 is directed downwards; and by the limitation of its legs to two pairs, of 

 which the posterior does not make its appearance outside the shell, being 

 bent upwards to give support to the ovaries. The valves are generally 

 opened widely enough to allow the greater part of both pairs of antennae 

 and of the front pair of legs to pass-out between them; but when the ani- 

 mals are alarmed, they draw these members within the shell, and close 

 the valves firmly. They are very lively creatures, being almost constantly 

 seen in motion, either swimming by the united action of their foot-like 

 antennae and legs, or walking upon plants and other solid bodies floating 

 in the water. Nearly allied to the preceding is the Cy there, whose body 

 is furnished with three pairs of legs, all projecting out of the shell, and 

 whose superior antennae are destitute of the filamentous brush; this genus 



1 Certain points of resemblance borne by Pycnogonida to Spiders, makes the 

 careful study of their development a matter of special interest and importance; 

 as there is some reason to regard them rather as Arachnida adapted to a marine 

 habitat, than as Crustacea. See Balfour's " Comparative Embryology," pp. 448, 

 449, and the authorities there referred to. 



