No. 2.] Ajvatoa/v of bdellodrilus illuminatus. 50 r 



it is secreted into all the external folds, and the oral, anal, and 

 sexual invaginations. It is transparent, colorless, non-irides- 

 cent, and under low powers apparently homogeneous ; and 

 maintains on the exterior a uniform thickness of .0015 mm. 

 By prolonged maceration in water it may be easily separated 

 as a continuous piece much larger than the body which it en- 

 veloped, and with the tubular invaginations all everted. Sur- 

 face views of such preparations show very distinctly the minute 

 perforations and the cruciform markings which so generally 

 characterize the cuticle of Oligichaeta ; and which Voigt has 

 described for Branchiobdella. Into the oral invagination and 

 through the pharynx the cuticle extends with undiminished 

 thickness, to terminate a short distance within the first 

 post-cephalic somite, after thinning to an excessively delicate 

 ragged edge. Conspicuous dorsal and ventral hollow thicken- 

 ings in the anterior region of the pharynx form the jaws. The 

 dorsal jaw is a medium longitudinal ridge bearing on the free 

 compressed margin three posteriorly directed teeth. This is 

 borne by a cuticular plate which anteriorly passes into a pair 

 of slightly divergent ridges, and posteriorly forms a single 

 median piece, passing on all sides into the unthickened cuticle. 

 The entire structure is molded on an epithelial thickening, over 

 which the supporting plate is bent at an obtuse angle, the 

 limbs of which embrace the muscular supporting pad anteriorly 

 and posteriorly, and afford points of attachment for the pro- 

 tractor and retractor muscles (Fig. 9, dj). The maximum 

 elevation of the dentigerous ridge is .01 mm., which is about 

 twice the actual thickness of the cuticle at this point. The 

 ventral jaw consists of a pair of similar parallel ridges, united 

 posteriorly by a heavy cross bar, and passes anteriorly, after 

 diminishing in thickness, into the unmodified cuticle. Each 

 ridge bears at its most prominent point a strong claw-like 

 tooth. The muscular pad supporting the ventral jaw is em- 

 braced in a manner similar to the dorsal one (Fig. 9, vj). The 

 mechanism of the jaws is explained below. There is a very 

 short anal extension of the cuticle, which reaches into the in- 

 testine scarcely to the anterior border of the anal segment. 

 No cuticular lining is apparent in the ovipore, and only a very 



