STRUCTURE OF SHELL. 35 



and certain genera of the Lepadideo. Thirdly, the scuta and 

 terga in Pollicipes, so strikingly resemble in manner of 

 growth — in position relatively to the animal's body — in 

 shape — and even in being articulated together, the valves 

 which form the operculum or lid of sessile Cirripedes, that 

 their identity is at once obvious. 



It may be well here further to premise, that apparently 

 none of the sutures in the shells of Cirripedes correspond 

 with the articulations between the three archetype cephalic 

 segments, of which the whole shell is formed ; or with the 

 eight elemental pieces, of which each separate segment in the 

 archetype crustacean is known to consist. But, as I believe, 

 the several valves in the shell of a Cirri pede are homo- 

 logous, or at least analogous, with the sclerodermic plates,"* 

 of which the carapace of the Podophthalmia is formed ; 

 with this difference, that in the latter they become, after 

 their first formation, united together into a single piece, 

 and are thus moulted as a whole; whereas in Cirripedes, 

 the valves or sclerodermic plates are not moulted, but 

 continue to be added to throughout life. 



In Pollicipes, there is no difficulty in understanding the 

 growth of the lower valves of the capitulum, especially if a 

 species be taken in which these valves stand a little way 

 apart : at each period of growth, they are added to at their 

 basal edges and a little way up both sides; at the same 

 time, a new membrane connecting them together is formed, 

 the old membrane disintegrating, or being left hanging in 

 tatters to the last zone of growth. Now if w r e look at 

 the shell of a sessile Cirripede, there is no essential 

 difference in the growth of the compartments or valves ; all 

 grow downwards and laterally; but they overlap each other 

 much more laterally than in Pollicipes, and the connecting 

 membrane is in most parts reduced to a mere film jammed 

 in between the valves ; but, in the case of the opercular mem- 

 brane, it still remains wide, and is periodically moulted. 



In the annexed woodcut (fig. 1), of the rostrum of 

 Balamis Ilamcri, the downward growth and the lateral 



* Milne Edwards, 'Annates des Sciences Naturellcs,' torn, xviii, (1852), 

 p. 230. 



