460 CIKBHOPODA. 



solely to attach the animal to rocks, stones, &c. ; and posteriorly, six 

 pairs of natatory members, so articulated as to act in concert, and to 

 give a very forcible stroke to the water, causing the animal, when 

 swimming, to advance by a succession of bounds, after the same manner 

 as the "Water-flea (Daphnia) and other Honoculi, but particularly 

 Cyclops, whose swimming-feet are extremely analogous. The tail, which 

 is usually bent up under the belly, is short, composed of two joints, and 

 terminates in four seta3, forming an instrument of progression. The 

 animal, moreover, is furnished with large pedunculated eyes. After 

 keeping several of the above for some days in sea-water, they threw off 

 their exuviae, and, becoming firmly adherent to the bottom of the vessel, 

 were changed into young Barnacles ; and the peculiarly-formed shells, 

 with their opercula, were soon distinctly formed, while the movements 

 of the cirri, although as yet imperfect, were visible. As the shell 

 becomes more complete, the eyes gradually disappear, the arms become 

 perfectly ciliated, and an animal originally natatory and locomotive, and 

 provided with a distinct organ of sight, becomes permanently and im- 

 moveably fixed, and its optic apparatus obliterated. 



(1186.) Similar results were obtained by watching the development 

 of the pedunculated type of Cirripeds* (Lepades), many of which were 

 proved in their earliest form to resemble different kinds of Monoculi, and 

 to be possessed of the capability of locomotion. 



(1187.) The manner in which larvae thus constituted are converted 

 into the fixed and pedunculated Cirriped is, indeed, one of the most re- 

 markable features connected with the history of the class. The larva in 

 its last stage has much the appearance of one of the Stomapod Crusta- 

 ceans ; and, as is the case in several genera belonging to that order, the 

 part of the head bearing the antennae and organs of sense in front of the 

 mouth equals or even exceeds in size the posterior part of the body, 

 consisting of the enclosed thorax and abdomen. On the borders of the 

 carapax at the anterior end, on the sternal surface, there are two 

 minute orifices, sometimes having a distinct border round them, within 

 which are contained minute sacculi, regarded as acoustic organs ; and, 

 moreover, large compound eyes, each consisting of eight or ten lenses, 

 are situated near the bases of the antennae. 



(1188.) But it is the antennae themselves that principally claim our 

 notice, inasmuch as it is by means of these organs that the creature 

 ultimately attaches itself when about to assume its complete or fixed 

 condition. They consist of three segments ; the first, or basal one, is 

 much larger than the others f, and apparently always has a single spine 

 on its outer distal margin. The second segment consists either of a 

 large, thin, circular sucking-disk, or is hoof-like ; in all cases it is fur- 

 nished with spines on the exterior hinder margin. The third and ulti- 

 mate segment is small ; it is articulated on the upper surface of the disk, 

 * Phil. Trans, for 1835, p. 355. f Darwin, loc. cit. 



