CIKEIPEDIA 



967 



really so abrupt as it might at first sight appear to the group of 

 Cirripedia, consisting of the barnacles and their allies ; for these, 

 like many of the Suctoria, are fixed to one spot during the adult 

 portion of their lives, but come into the world in a condition that 

 bears a strong resemblance to the early state of many other 

 Crustacea. The departure from the ordinary crustacean type in 

 the adults is, in fact, so great that it is not surprising that zoolo- 

 gists in general should have ranked them in a distinct class, their 

 superficial resemblance to the Mollusca, indeed, having caused most 

 systematists to place them in that series, until due weight was 

 given to those structural features which mark their * articulated ' 

 character. We must limit ourselves, in our notice of this group, 

 to that very remarkable part of their history, the microscopic 

 study of which has contributed most essentially to the elucidation 



FIG. 721. Development of Balanus balanoides : A, earliest 

 form ; B, larva after second moult ; C, side view of the same ; 

 D, stage immediately preceding the loss of activity; a, 

 stomach (?) ; b, nucleus of future attachment (?). 



of their real nature. The observations of Mr. J. Y. Thompson, 1 with 

 the extensions and rectifications which they have subsequently 

 received from others (especially Mr. Spence Bate 2 and Mr. Dar- 

 win 3 ), show that there is no essential difference between the early 

 forms of the sessile Cirripeds (Balanidce or 'acorn-shells') and of the 

 pedunculated (Lepadidce or ' barnacles ') ; for both are active little 

 animals (fig. 721, A), possessing three pairs of legs and a pair of 

 compound eyes, and having the body covered with an expanded 

 carapace, like that of many entomostracous crustaceans, so as in no 



1 Zoological Besearches, No. IV. 1830, and Phil. Trans. 1885, p. 355. 



2 'On the Development of the Cirripedia' in Ann. Nat. Hist. ser. ii. vol. viii. 

 1851, p. 324. 



5 Monograph of the Sub-Class Cirripedia, published by the Ray Society. 



