56 Prof. Ehlers on the Vermes 



that one of the tentacles bore the disciform operculum ; but 

 during my endeavours to ascertain precisely the position of 

 this specialized tentacle among the rest, it separated, with a 

 number of the neighbouring filaments, and I could only make 

 out with certainty that it had its place not in the median line 

 but in the right half of the bundle of tentacles. On the 

 isolated tentacle there could be distinguished the piece (11 

 millims. in length) by which the filament was attached to the 

 cephalic lobe among the others, the operculiform circular 

 plate effecting the closure (with a superficial diameter of 3 

 millims.), and a filiform piece (2*5 millims. in length) which 

 projected freely from the centre of the outer surface of the 

 operculum. With the aid of the microscope it was ascertained 

 that the operculum was composed of the same tissues as the 

 filament ; but the mode in which the opercular disk may be 

 formed from the filament is not quite clear to me. On the 

 surfaces of the disk there was a chitinous cuticle, such as 

 occurs on the tentacles ; and this was evidently in connexion 

 with the above-mentioned sections of the tentacular filament 

 on each surface in the centre of the disk ; and thus it might 

 seem as if the disk were formed by a duplicature of the wall of 

 the tentacle, perhaps by a portion of the tentacle being com- 

 pressed discoidally by a pressure acting in the direction of its 

 longitudinal axis. In the space between the two lamellae 

 forming the disk there were, besides a small quantity of con- 

 nective tissue, a number of spherical bodies, which, perhaps, 

 are to be regarded as corpuscles of the body-fluids. The 

 point in connexion with the structure of the disk into which 

 I could not get a clear insight was, that in the interior of the 

 disk and closely approximated to its margin there was a short 

 filament, which fell out when I removed the chitinous cuticle 

 of one of the surfaces : in its appearance tjbe filament re- 

 sembled a piece of tentacle ; one of its extremities was appa- 

 rently intact, and the other evidently injured, as if the little 

 filament had been here torn away. I must leave the more 

 accurate investigation of this opercular disk to others, who 

 may have more abundant material at their command. Here 

 I would only call attention to one or two points. In the first 

 place the evident homology which exists between this oper- 

 culum of a Terebellacean and that of a Serpulacean ; in both 

 cases one of the appendages issuing from the cephalic part is 

 modified in such a manner as to form an operculum ; and 

 although the thin opercular disk of our Scione does not attain 

 the development of the strong operculum of the Serpulaceae, 

 on the other hand it displays the peculiarity of possessing, in 

 the little filament which springs from the outward wall, a 



