432 THE MICROSCOPE. 



species of marine shells. The shell forms a short tube, and is 

 usually composed of six segments securely united together. 

 The lower part of this tube is firmly fixed to the object on 

 which the Balanus has taken up its abode; whilst the 

 superior orifice is closed by a moveable roof, composed of 

 from two to four valves, between which the little tenant 

 of this curious domicile protrudes his delicate cirri in 

 search of nourishment. In the young state the Balanidce 

 freely swim about and somewhat resemble the following 

 group, the Entomostraca. 



Entomostraca occur in countless swarms in all waters, 

 whether salt or fresh ; and, minute as they are, one of the 

 species is said to constitute the principal food of the Whale. 

 The genus Cyclops (fig. 215), specimens of which may be 

 found in every stagnant pool, as well as the open seas, the 

 type of the family Cydopidce, is characterised by the pos- 

 session of a single eye. In the Cetochilidce there are two 

 of these organs. 



The animals comprising the order Ostracoda are generally 

 of very minute size ; the body, which strongly resembles 

 that of the Copepoda, is always enclosed in a little bivalve 

 shell, the feet and antennae being protruded between the 

 lower edges of the valves. These little shells so closely 

 resemble those of minute bivalve Mollusca, that those of 

 some of the larger species have actually been described by 

 conchologists as the coverings of animals belonging to that 

 class. The antennae are often curiously branched; and 

 the hinder extremity is usually prolonged into a sort of 

 tail, which is seen in constant action when the animal is 

 in motion. In Cypridina, the body is entirely enclosed by 

 a shell, of which the genus Cypris (fig. 215) is an example; 

 and in Daphnia, " Water-fleas/' the head is protruded be- 

 yond the shell. In Polyphemida the head is large/ and 

 almost entirely occupied by an enormous eye, giving the 

 creatures a most singular appearance; the Monoculus is 

 a well-known example of this group. Another family, not 

 provided with a shell or carapace, called Branchiopoda, 

 from the name of the typical genus, Branchiopus stagnates 

 (fig. 215), is often found after heavy rains in cart-ruts and 

 other small pools. The Artemia salina inhabits a still 

 more curious situation, namely, the salt-pans at Lyming- 



