40 NATURAL ARRANGEMENT OP INSECTS. 



ance of the valves is seen, and also an elbow of the 

 anterior members of the animal, the tail part being 

 the narrowest. When the animal is very highly mag- 

 nified,, and all its parts seen, its limbs are protruded 

 from the anterior opening of the valves : these consist 

 of the two fore feet, which have a peculiar position when 

 they are extended by the animal, in order to fix itself 

 by means of the sucker and claw at the end : a fourth 

 large basal joint remains concealed by the shell; towards 

 the other extremity of which, and placed beneath, are 

 six pairs of swimming legs, used in the manner of oars 

 to propel the animal forward. At the extremity of the 

 tail are two, short, setiform appendages, which no doubt 

 act as a rudder. The appearance of the eye, as seen 

 through the shell, gives an inaccurate idea of its form; 

 but, when highly magnified, its pedunculated shape 

 clearly appears. There is an appearance on the back of the 

 shell, which Mr. Thompson considers as the presumed 

 nucleus of future attachment. The tail, when highly 

 magnified, is seen to consist of two articulations, from 

 which spring four setae, or bristles, two being shorter 

 than the others : the swimming feet, also, when very 

 highly magnified, are then seen to consist of two divi- 

 sions, the shortest of which is that which comes 

 nearest the body : these are the organs, in fact, which 

 are changed into the cirrhi of the barnacle. The natural 

 size of the animal, after its metamorphosis into a bar- 

 nacle, is not larger than that of the larva state. When 

 highly magnified, the rudiments of the eyes can still 

 be traced through the large valves of the operculum, 

 and the enclosed animal is still seen through the central 

 opening of the operculum. The valves of the body of 

 the shell are marked, as before observed, by distinct 

 sutures. When viewed in profile, the arms, or cirrhi, 

 if protruded, become more conspicuous. One of these 

 cirrhi, upon being very highly magnified, although 

 much jointed, did not at first exhibit any appearance 

 of fringed hairs. The full-grown barnacle is seldom 

 more than half an inch long : when viewed in front, 



