CEMENTING APPARATUS. 151 



the structure seemed to be essentially the same. In BaL 

 galeatus, I found the cement-ducts varying in diameter 

 from 3555th to 7o ^th of an inch in diameter. In B. improvisus, 

 the cement-glands clo not differ much from those of 

 B. tintinnabulum ; but the cement-ducts bifurcate often 

 before entering the circumferential duct; and the little 

 branches, which proceed from the latter, are very short, 

 and almost immediately, owing to the thinness of the basis, 

 blend into a slip of cement. 



I hope to be excused for describing at such length, the 

 apparatus by which sessile cirripedes are permanently 

 attached to a supporting surface; for this is the great 

 leading character of the sub-class, not hitherto observed in 

 any other Crustacean.* It is not easy to overstate the 

 singularity and complexity of the appearance of the basal 

 membrane of a Balanus or Coronula : and when we con- 

 sider the homological nature of the apparatus, the subject 

 becomes still more curious : I feel an entire conviction, 

 from what I have repeatedly seen in several genera of the 

 Lepadidse, both in their mature and pupal condition, and 

 from what I have seen in Proteolepas, that the cement-glands 

 and ducts are continuous with and actually a part of an 

 ovarian tube, in a modified condition ; and that the cellular 

 matter which, in one part, goes to the formation of ova 

 or new beings, in the other and modified part, goes to the 

 formation of the cementing tissue. To conclude witli an 

 hypothesis, — those naturalists who believe that all gaps in 

 the chain of nature would be filled up, if the structure of 

 every extinct and existing creature were known, will readily 

 admit, that Cirripedes were once separated by scarcely 

 sensible intervals from some other, now unknown, Crusta- 



* Rathke lias described ('Acta Nova,' 1839, p. 147), in some siphouosto- 

 matous crustaceans, a pair of curious organs, which serve to secrete a sub- 

 stance that holds the eggs attached together in a mass to the parent's body : 

 these organs Rathke has designated by a similar name to that which I have 

 used, namely, the cementing organs or receptacles ; they are distinct from the 

 oviducts, but enter them near their external orifices. As in Cirripedes, the 

 cement-glands and ducts are certainly continuous with an ovarian tube ; and as 

 they occupy a quite different position in the animal's body, these organs of 

 Rathke, though in some degree analogous in function, must be homologically 

 distinct. 



