THE ANAL GLANDS. 207 



latter with the glandular tube becomes drawn out into a short canal 

 (Fig. 103 B), which enters the latter dorsally (Sedgwick). 



The two external apertures of the nephridia (Fig. 103, sp) are 

 displaced into the buccal cavity by the fold which encloses the 

 mouth. They here come to lie in a transverse groove, which, as 

 the buccal cavity develops further, becomes deeper and shorter. 

 This groove eventually becomes a short canal with a slit-like lumen, 

 into which the two nephridial canals (salivary glands) open. This 

 is the common efferent duct of the salivary glands opening into the 

 buccal cavity (v. Kennel). 



The Anal Glands. 



The so-called anal glands, a pair of glandular tubes which, in the 

 male of P. Edwardsii, open ventrally on either side of the anus, and, 

 in P. capensis, open through a short, common efferent duct at the 

 genital aperture,* and are evidently related to the genital apparatus, 

 are shown by their development to be modified nephridia (v. Kennel). 

 They arise in P. Edwardsii from the primitive segments of the last 

 (limbless) segment upon which the anus opens ventrally. The anal 

 glands occur as rudiments in both sexes ; in the male only, however, 

 do they attain the functional tubular form ; in the female they 

 degenerate. 



In P. capensis, at the male genital aperture, a pair of glands open which are 

 apparently the homologue of the anal glands of the American species. But 

 since the nephridia of the segment which carries the genital aperture give rise 

 to the efferent ducts of the genital apparatus (see below, p. 209), these glands 

 must have a different origin. It appears probable that they are derived from 

 one of the two additional pairs of primitive segments found by Sedgwick in 

 P. capensis behind the primitive segments of the anal papillae. In this form, 

 the genital aperture has shifted to a position quite near the anus, lying in front 

 of it on the segment carrying the anal papillae. In P. Edwardsii, on the 

 contrary, the genital aperture is found two segments further forward, on the 

 penultimate limb-bearing segment. Since, according to Sedgwick, there are 

 still two segments which remain in an undeveloped condition behind the last 

 fully formed primitive segment (that of the efferent genital ducts), it might be 

 assumed that these corresponded to the last limb-bearing segment and to the 

 so-called anal segment of the American species. The latter would thus have two 

 well-developed segments (the genital segment and that following it) in a region 

 where in the African and New Zealand species a degeneration occurred, which led 

 to the genital and anal apertures coming to lie on apparently one and the same 

 segment. This would also explain the approximation of one of the last pairs 

 of nephridia (the anal glands) to the antepenultimate pair (the efferent genital 

 ducts). This assumption seems to be confirmed by the fact recently made 



* [In P. novac-britanniae (Willet), the pygidial (anal) glands open by a 

 median aperture situated immediately above the anal orifice. — Ed.] 



