212 AKTHROPODA [CH. 



part (usually the exopodite) and by the development and modification 

 of others, the original form becomes masked and difficult to recognise. 

 When both forks are conspicuously developed the limb is said to be 

 biramous. The last four or five segments of the Copepod's body 

 bear no appendages. The last is produced into two processes, forming 

 a caudal- or tail-fork. 



The sexes in the free-living species are not markedly different, 

 but if we examine specimens of such a genus as Cyclops, which is 

 common in our fresh- water pools, we shall find that in the breeding 

 season the female carries about with her two egg-sacs (Fig. 82). 

 These are attached to her body just behind the last pair of append- 

 ages and project freely at the side. Each egg-sac may contain four 

 or five dozen eggs which are glued together by a cement-substance. 

 Such egg-sacs are very characteristic of the Copepoda and are 

 found even in the parasitic members of the order. Most of the 

 latter live on fish, and some have acquired the name of "fish-lice." 

 Their mouth appendages have lost their biting function and have 

 become adapted for piercing the tissues of the host on which they 

 live. Their segmentation is suppressed and their appendages are 

 reduced and the body has grown out into all sorts of curious 

 processes. The male is often much smaller than the female and 

 as a rule retains the crustacean characters more than she does. 

 Occasionally they are found on the skin of a fish, but more often 

 they occur in the mouth and on the gills, sometimes half and 

 sometimes wholly embedded in the flesh of their host. 



Order IV. Cirripedia. 



Some of the Copepods have become so modified by their parasitic 

 habits that unless we were able to trace their development, in- 

 cluding the larval forms through which they pass before becoming 

 adult, we should have difficulty in assigning them to their proper 

 place amongst the Crustacea. A somewhat similar modification 

 occurs in the Cirripedia (Lat. cirrus, a tuft of hair ; pes, a foot) and 

 is associated with a fixed or sessile habit of life. After passing 

 through a variety of free-swimming larval forms the animal comes 

 to rest and attaches itself by the anterior end of the body to a stone 

 or rock, the bottom of a ship, or some other object submerged in 

 the sea, and then becomes adult. 



Like that of the Ostracoda the body of a Cirripede is enclosed 

 in a carapace consisting of two valve-like folds which have grown 

 out from the region of the head, but these are usually strengthened 



