PHYLUM ARTHROPODA 215 



Cypris is free-swimming, while Candona is of a burrowing habit. 

 In the marine forms (Cypridinidae) a heart is present but in 

 fresh- water ostracods there is no heart. 



Subclass ClRRIPEDIA 



Most Crustacea are free-moving animals except the parasitic 

 forms and the members of the entire group known as the Cirri- 

 pedia or barnacles. Not only are the barnacles sessile but as a 

 consequence of this habit they show very marked degeneration 

 of many structures. Adaptation to the sessile life also helps 

 to explain the fact that hermaphroditism, which is so uncommon 

 in Arthropods, is the usual condition among the Cirripedia. 

 Though occasionally males are found, they are usually small, 

 degenerate forms known as complemental males which very 

 frequently live within the shell of the female, at times as parasites. 



The body is enclosed in a membranous mantle which, in most 

 instances, is encased in calcareous plates. It is not surprising 

 that until less than a century ago barnacles were thought to be 

 molluscs. 



In development, the Cirripedia hatch from the egg as a naup- 

 lius. Because of its resemblance to the ostracod, Cypris, a 

 second larval stage with a bivalve shell, is termed the cypris stage. 

 The free-swimming larva comes into contact with some object 

 to which it becomes attached. The first antennae, with their 

 associated cement glands, aid in the fixation. Calcified plates 

 usually make their appearance in the mantle folds and form a 

 protective shell surrounding the animal. Within this shell, the 

 body of the barnacle is peculiarly oriented, for, as is sometimes 

 said, the barnacle stands on its head and kicks food into its mouth. 

 Water currents bearing food to the mouth are set up by the 

 beating of the usually six pairs of plumelike biramous appendages 

 of the trunk. The mouth is surrounded by a pair of mandibles 

 and two pairs of maxillae. 



The goose barnacles {Lepas anatifera) have the region of 

 attachment (Fig. 100) drawn out into a characteristically 

 elongated stalk. A mediaeval myth maintains that the goose 

 develops from these goose barnacles. The neckless barnacles 

 (Balanus, for example), which encrust rocks and other submerged 

 objects, have a flattened base of attachment from which the 

 skeletal plates arise directly. Barnacles are exclusively marine 

 and choose stones, wood, animals, plants — ^in short, any object — 



