60 BALANllXffi. 



slips of membrane left adherent on the opercular valves, 

 sheath, and walls, stand in rows; a row corresponding to each 

 period of exuviation of the opercular membrane. The bristles 

 are generally largest on the opercular valves and sheath ; 

 in Balanus tintinnabulum, they are from j^Jths of an inch 

 in length, but they are longer in some other species. I 

 may here mention, as showing the connexion of these 

 bristles with the opercular membrane, that similar bristles 

 occur in B. perforatus, scattered over the surface of that 

 membrane, and are necessarily moulted with it. In the 

 imbedded genera Coronula and Tubicinella, none of these 

 bristles exist. When a portion of valve or shell, fur- 

 nished with bristles, is dissolved in acid, tough, sinuous, 

 and apparently hollow, threads are seen to run from 

 their bulb-like bases, into and up the corresponding layer, 

 which, before dissolution, existed as shell ; and they 

 terminate internally in very fine points, which I believe 

 are united to the underlying corium. These threads, or 

 tubuli* as I have called them in my volume on the Lepa- 

 didse, are, in Tetraclita porosa, about ^ths of an inch in 

 diameter, but only half that size in B. tintinnabulum. On 

 parts of the shell where there are no bristles, similar tubuli 

 penetrate the shelly layers, and come to the surface. The 

 tubuli running to the lowest and last-formed row of bristles, 

 just after a period of exuviation, are so delicate as hardly, 

 or not at all, to be distinguished ; in the row above, they 

 are plain and longer, and for the next two or three upper 

 rows they are, in some cases, as in Tetraclita jjorosa, 

 longer and longer, having been added to during each suc- 

 cessive thickening of the valve. These tubuli consist of 

 chitine, and no doubt first existed as threads of corium ; 

 they are so tough that they must serve to strengthen 

 the successive layers of shell, but I imagine their chief func- 

 tion is to keep up the vitality of the newly- formed layers 

 of shell. May we not, also, venture to suppose that by their 

 means, some degree of sensibility is given to the bristles ? 



* I regret that I have used this term "tubuli"; for the threads thus desig- 

 nated, I believe, are not the same with the tubuli of Dr. Carpenter, which are 

 not left after dissolution in acid. I have seen tubuli, as called by me, in the 

 shell from the leg of a crab, after having been placed in acid. 



