280 



THE MICROSCOPE. 



sort of mantle which lines the interior of the shell. The cirri are 

 twelve in number, and beset with bristles. When the animal is alive 

 they may be seen i,n continual motion, exserted 

 and retracted every moment in search of prey. 

 The intestinal canal is complete, furnished with 

 a mouth and an anal opening ; and the nervous 

 system exhibits the usual series of ganglia, 

 Tvhich we have seen to be characteristic of the 

 articulate type. The head is marked only by 

 the position of the mouth, which is armed 

 with a pair of jaws ; but all traces of any of 

 the organs that we are accustomed to see at 

 this part of the body have completely disap- 

 peared. 



The second family, the Balcmidm or sea- 

 acorns, includes the sessile species, whose cu- 



fig. 121. 



1. Young fry of the Oyster, a . i..,t i i •/ ,• j_ ii i ^ 

 portionofthemnearlyready "^^^ ^'■^''^^ habitations may constantly be met 

 to escape from the shell, with upon the rocks of the sea-shore, and not 



2. Body andshell of Barnacle, unfrequently upon many species of marine 

 shells. The shell forms a short tube, usually composed of six segments 

 firmly united together. The lower part of this tube is firmly fixed to 

 the object on which the Balanus hag taken up its abode ; whilst the 

 superior orifice is closed by a movable roof, composed of from two to 

 four valves, between which the little tenant of this curious domicile 

 can protrude his delicate cirri in search of nourishment. In their 

 young state the, Balanidse resemble the following group, the Ento- 

 mostraca. 



These animals occur in countless swarms in all waters, whether salt 

 or fresh ; and, .minute as they are, one species is said to constitute the 

 principal food of the whale. 



The best known form is the genus Cydops (fig. 122), specimens of 

 which may be found in every stagnant pool ; it is the type of the 

 family Cyclopidce, characterised by the possession of a single eye. In 

 the CetochilidcB 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 Gopepoda, 

 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 Moll^sca, that those of 

 some of the larger species have actually been described by eonchologists 

 as the coverings of animals belonging to that class. The antennae 



