INTESTINAL CANAL OF THE ICHTHTOPSIDA. 397 



typical examples, drawn to the same scale relative to the trans- 

 verse diameter of the base of the adjacent intestine (a — (3 of PI. II. 

 fig. 14). These structures show, so far as I have been able to 

 ascertain, little sign of individual variation : the first noticeable 

 features concerning them are (i.) that the length of the duct bears 

 little or no proportion to either the length or bulk of the gland — 

 the latter is most massive in Lcemargus (dv", fig. 10), where the 

 duct (dv) is shortest ; and (ii.) that the maximum attenuation of 

 the gland is not accompanied by that of the duct (cf. Acanthias, 

 fig. 13, and Notidanus, fig. 14). This " duct " has been pre- 

 viously best described by Home* and Blanchard. The last- 

 named author regards it {Acanthias, 2. p. 182) as a duplicature 

 of the intestinal mucous membrane. It is more or less marked 

 in all the Selachoidei which I have been able to examine ; and it 

 will be seen from the accompanying figures that it attains its 

 greatest attenuation in Notidanus (fig. 14); between the con- 

 ditions exemplified in this genus and in Lcemargus (fig. 10), 

 taken as extreme terms in the series, gradational types present 

 themselves in Scyllium, Cestracion, and Acanthias (figs. 11, 12, 

 13). On examination of these, the duct in question (dv) might 

 readily appear to have been formed by a downward extension of 

 a simple fold of the intestinal wall, such as that of Laemargus 

 (fig. 10). Lcemargus is remarkable for the possession of the 

 most aberrant type of intestinal canal met with in living Pla- 

 giostomes. Its duodenal segment is, unlike that of all other 

 Elasmobranchs, tubular and flexed, and the bearer of a couple 

 of enormous diverticula which Turner has (31. p. 245) compared 

 to the pyloric caeca of the Osteichthyes ; its appendix digitiformis 

 (fig. 10) is the most massive that has yet been observed ; and, in 

 view of the facts just named, it becomes doubtful whether that 

 structure may not be in a much modified condition. 



In the Latoidei the processus digitiformis communicates with 

 the intestine by a short non-constricted passage {dv', fig. 1), 

 little suggestive of the " duct " of the Selachoids. Monro has 

 already figured this in Baia t, and I find an identical condition 

 in Torpedo, Trygon, and the rare LLypnos suhnigrum. Com- 

 parison of figs. 1 and 3 at once suggests an homology between 



* He naively likened the apparatus (15. p. 392 [a. d. 1814]) "to the ink- 

 bag iu the Cuttle-fishes " {cf. his 16. pi. xcviii.). 



t 23. pi. ix. fig. 2, 

 LINN. JOUEN. — ZOOLOGY, VOL. XXIII. 28 



