254 DR M'INTOSH ON SOME POINTS IN THE STRUCTURE OF TUBIFEX. 



from the Thames, they have never occurred in those found in this neighbour- 

 hood. The number of pairs of bristles in front ranges from twenty-one to 

 twenty-three. The hooks are gently curved organs, with a bifid tip, and a dis- 

 tinct swelling or shoulder about the upper third, from which point they taper 

 towards the base. Those accompanying the bristles anteriorly (Plate IX. fig. 1) 

 slightly differ in their curvature from those of the ventral series (Plate IX. fig. 2). 

 In the other and longer form (with about 150 segments) from the lakes, the 

 fourth segment has a pair of bristles, and the latter increase in length till the 

 twelfth segment is reached, after which they gradually diminish and disappear 

 about the fortieth. There is a small papilla where each bristle-bundle passes 

 through the skin, and the tips of the hairs are delicately serrated or roughened, 

 the serrations being directed distally. In this form the bristles are larger than 

 the diameter of the body, and hence it differs from Nais scotica and the Nais 

 lacustris of Dalyell. The hooks commence with the bristles, and besides 

 those accompanying the latter, form two rows, as usual, inferiorly, which rows 

 in front consist of groups of four hooks. Those accompanying the bristles 

 (Plate IX. fig. 3) are more closely forked at the tip, and if examined under a 

 power of 700 diameters show certain processes in the fork (a), a fact first 

 pointed out to me by Mr Lankester, whose larger specimens from the Thames 

 exhibited this and other peculiarities in a marked manner. These are also 

 less shouldered, less curved, and somewhat more elongated than the inferior 

 hooks. The latter in each form of Tubifex continue after the last bristle- 

 bundle, and thus form four rows posteriorly, the terminal segment only being 

 bare. M. d'Udekem's representations of the hooks,"" though easily recognisable, 

 deviate a little from the foregoing, and the same may be said of M. Claparede's 

 figures of the hooks of his Tubifex papillosus.f 



Body- Wall. — M. d'Udekem speaks of the epidermis as being separable from 

 the chorion by the aid of an alkaline solution, but I have not been able to dis- 

 engage it as a distinct layer either by the action of chemicals in the fresh animal, 

 or in transverse sections of the body- wall. M. Claparede does not mention 

 the superficial layer as a distinct coat, but groups it with the subjacent, under 

 the name of cuticle.! The cuticular surface (or layer) is quite homogeneous, 

 but the chorion which is incorporated with its inferior surface is distinctly 

 cellulo-granular, as described by Gruithuisen in Nais (Chcetog aster), and by 

 Buchholz in Enchytraeus. This is most distinctly marked at the snout and 

 tail, where the layer is thickened. The other layers in Tubifex are a belt of 

 circular muscular fibres, and a longitudinal muscular coat. M. Claparede 

 gives an ideal section of Limnodrilus, a form which differs from Tubifex in the 



* Op. cit. PI. II. figs. 6 and 7. 



t Beobach. fiber Anat. u. entwicklung. &c, p. 25, Pi. XIII. fig. 15. 



£ Rechercbes Anat. sur les Oligocbetes, p. 7. 



