ON TUBIFEX. 151 



Tubifex. The cuticular and cellular layers of Tiihifex are best seen 

 in the glandular clitellus which surrounds the generative segments. 

 At the period of maturity, these cells acquire a far greater 

 development than those of other parts ; they swell out and distend 

 the cuticle from the muscular layer, and many of them are character- 

 ised by a granular appearance, as shown in Plate i6, Fig. i. 



It will be remembered that I have described the setse as being 

 placed in pouches or invaginations of the epidermis. * D'Udekem 

 also describes them as thus placed, but further examination leads 

 me to reconsider this statement There is, I think, a common 

 envelope surrounding the basis of all the setae in a fascicle, and it 

 gives strongly the impression of an invagination of the integument- 

 ary layers ; but on carefully observing the movements of the setse 

 this impression is very much weakened, for they do not seem at all 

 to issue from a common orifice, but each appears to penetrate the 

 cuticle by a separate, very minute aperture or pore. If we con- 

 ceive a piece of board in which are five or six holes placed in a 



row, thus I o o o o o I and each hole having a stick passed through 

 it. Now, if we bring together the ends of the sticks thus protruding 

 and grasp them with the hand and move them about, we shall 

 have a fair idea of the movement of the set^ in a fascicle. Within 

 the common envelope each seta is, I think, enclosed in its own 

 separate cell, of which it is a secreted product, in the same way 

 as the cuticle is regarded by Gegenbaur as the secreted product of 

 the epidermic cells, f 



That this 'is, indeed, the case with the new setae which one 

 constantly meets with in course of formation beside the older ones 

 of the fascicle, is, I think, certain, as the cell surrounding the new 

 seta is rendered visible from the more dense and granular 

 character of its contents, and its light-brown colour. Within this 

 cell, the new seta may be seen, at first only its forked tip, and this 

 tip of the full size that it is to have. Subsequent growth takes 

 place by increment at the base ; the central swollen portion is 

 the next to appear ; and finally the basal portion gradually acquires 



* Vol. I., page 17 of this Journal. 



t Gegenbaur, Comp. Anat., Bell's Translation, p, 139, says: — The setoe 

 must be regarded as differentiations of the integument of the class of cuticular 

 formations. " 



