METAZOA CRUSTACEA. 381 



Packard 1 states that these so called "lines of growth" 

 are superficial like the tubercles and. spines on other 

 Crustacea, and that therefore they are probably a kind of 

 ornamentation. The shell is in reality a modification of 

 the carapace which bends downward and encloses the 

 whole body. The fleshy animal (fig. 2, with one valve 

 removed) has the segmented body and jointed append- 

 ages which prove that it does not belong to the class of 

 Pelecypods. The thoracic and abdominal segments are 

 similar and bear twenty-two pairs of feet. Most of these 

 are alike in structure, but the first two pairs in the male 

 are provided with claspers. The head segments are 

 soldered together. 



ClRRIPEDIA. 



Barnacles have undergone numerous modifications 

 which have carried them far from the primitive ances- 

 tral form. The young, however, is a free-swimming nau- 

 plius (PI. 814, fig. i) similar to that of other generalized 

 Crustacea. It possesses the two pairs of antennae and 

 the mandibles which are locomotor organs as in other 

 members of its class. After moulting twice it appears as 

 seen in fig. 2. It now has a segmented abdomen which 

 serves as a rudder and the cephalothoracic appendages 

 are larger. In. the still older larval stage (fig. 3, side 

 view), when the barnacle is preparing to settle, the parts 

 and organs which are useful in swimming become reduced 

 in size or are dispensed with altogether. Thus the abdo- 

 men is a mere vestige, while the compound eyes and one 

 pair of antennae, wholly disappear. The cement duct in 

 the remaining pair of antennae, which connects with the 

 cement gland in the stalk, pours out its cement whereby 

 the barnacle is fastened by the anterior part of its head 

 to a rock. 



1 Loc. cit., p. 377. 



