CEMENTING APPARATUS. 133 



find the carapace composed of sclerodermic plates, which, 

 though closely joined and only occasionally separated by 

 sutures, yet in their origin are distinct;* they tend, also, to 

 be arranged in alternate or tile-like order. As the animal 

 grows, the old sclerodermic plates, all joined together, are 

 moulted, and new ones, also all joined together, of a larger 

 size, are formed beneath. Now let us imagine the growth 

 to be more gradual but yet periodical, and the new and 

 larger sclerodermic plates, when formed under the old ones, 

 to adhere firmly to them ; the older plates would thus be 

 prevented from becoming confluent, and instead of being all 

 moulted together, as is now the case, they would be almost 

 continually separated from each other, owing to the almost 

 continuous increase in size of the new underlying plates. 

 Consequently lines of splitting would run between the 

 several plates, however numerous they might be, instead of 

 there being 5 as now, a single line of splitting extending 

 down the back. In fact, we should have the identical 

 manner of growth of the shell or carapace, which occurs in 

 Cirripedes. It is on this ground, and from the several 

 points of homological resemblance incidentally mentioned 

 in the last few paragraphs, that, in the early part of this 

 Introduction (p. 13), when discussing the whole class, I 

 stated that I believed that the carapace of Cirripedes pre- 

 sented more real resemblance with the carapaces of the 

 Podophthalmia, or higher Crustacea, than with those of the 

 lower Crustacea, though in mere shape they more nearly 

 resembled the latter. 



Cementing Apparatus (Plate 28.) 



I have already (p. 128) given an account of the manner in 

 which, in the pupa of Lepas,-the cement-tissue escapes 

 from the prehensile antennas, and of the structure of the 

 cement-ducts, and of the cement-glands or incipient 

 ovaria ; and likewise of the changes by which these organs 

 assume their ultimate form in the mature Cirripede. In 

 my former volume, on the Lepadidse, I described the 

 cement-glands and the cement-tissue in several genera, 



Annales cles Sciences Naturelles,' 3d series, torn, xvi, pp. 233, 236, 237. 



* i 



