EXUVIATION ; RATE OF GROWTH. 61 



sessile Cirripedes, and that in tins family they serve as 

 Branchiae. 



The above- described method by which Cirripedia lay 

 their eggs, namely, united together in a common mem- 

 brane, placed between their old outer and new inner 

 integuments, and the manner in which the lamellae, 

 when thus formed, are retained for a time fastened to 

 the fraena, and are then cast off, appears to me very 

 curious. In some of the lower Crustacea, it is known, 

 that the ova escape by rupturing the ovisacs formed 

 by the protruded ovarian tubes, and this is the nearest 

 analogy with which I am acquainted. The ova are 

 impregnated (as I infer from the state of the vesiculae 

 seminales), when first brought into the sack, and whilst 

 the membrane of the lamellae is very tender • the long 

 probosciformed penis seems well adapted for this end. 

 In the male of Ibla Cumingii, which has not a probosci- 

 formed penis, the whole flexible body, probably, performs 

 the function of the penis : in Scalpettum omatum, however, 

 the spermatozoa must be brought in by the action of the 

 cirri, or of the currents produced by them. That cross im- 

 pregnation may and sometimes does take place, I infer 

 from the singular case of an individual, in a group of 

 Balani, in which the penis had been cut off, and had 

 healed without any perforation ; notwithstanding which 

 fact, larvae were included in the ova. 



Exuviation; Mate of Growth; Size. — I have had 

 occasion repeatedly to allude to the exuviation of the 

 Lepadidae : with the exception of the genus Lithotrya,* in 

 which the calcareous scales on the peduncle, together 

 with the membrane connecting them, is cast off, neither 

 the valves nor the membrane uniting them, nor that 

 forming the peduncle with its scales and styles, are 



* The external integuments being moulted in Crustacea, but not in 

 the Cirripedia, may appear, at first, an important difference : but we here 

 see that non-exuviation is not universal amongst the Lepadidae, and, on 

 the other hand, according to M. Joly, ('Annales des Sciences Naturelles,' 

 2d series, Zoolog.), there is one true crustacean, the Isaura cycladoides, 

 which has a persistent bivalve shell. 



