INTESTINAL CANAL OF THE ICETHTOPSIDA. 395 



orifice just alluded to * ; while ia Gestracion it does so at a point 

 situated lineally with this below. 



That the spiral valve performs an absorptive function can 

 hardly be doubted after the investigations of Edinger (9. p. G78) 

 into its histological structure. That segment of the intestine 

 which bears it comes thus to represent most nearly the ileum f 

 of the higher Vertebrata ; and, inasmuch as the valve may extend 

 back to t1ie point of origin of the " duct " of the appendix digiti- 

 formis {dv , fig. 14), that portion of the gut lying immediately in 

 front of the latter can consequently be most satisfactorily com- 

 pared only with the small intestine. The application to it of 

 the term " rectum " is no longer justifiable ; and the above facts 

 warrant the restriction of the term " large intestine " to so much 

 of the gut as is situatad behind the duct referred to (= that 

 portion embraced by the lines radiating from i.l. in fig. 1). If 

 this be admitted, tlie comparison, before instituted {ante, p. 394), 

 between the appendix digitiformis and its duct and the appendix 

 vermiformis and caecum, becomes vastly strengthened, and these 

 two sets of structures may justly be alike regarded as median 

 diverticula of the antero-dorsal extremity of the large intestine. 



Parker's researches show (26), when looked at critically, that 

 the spiral valve varies, among Elasmobranchs generally, in 

 nothing more conspicuously than in its degree of abbreviation 

 from behind forwards. The belief in the primitive characters of 

 the living Notidanidce is becoming more and more justifiable 

 from the researches of palaeontologists % ; and we may therefore 

 attach a special importance to the great development of the 

 spiral valve in Septanclms (cf. ante), as it furnishes us with a 

 condition from which, so far at any rate as backward extension 



* Such does not appear to be the case in the allied Chlamydoselache. Cf, 

 Garman (10) and Giinther (11). 



t Home alluded to it (15. p. 391) as the "jejunum." It is exceedingly 

 unfortunate that in students' books current it should be spoken of as the 

 "colon" (Marshall and Hurst's 'Junior Course in Practical Zoology,' ed. 2, 

 1888, p. 219). 



X Cf. A. S. Woodward, P. Z. S. 1880, pp. 218-224, and Geol. Mag. dec. iii. 

 pt. iii. 1886, pp. 205 ut seq. 



Haswell has still further emphasized the belief, in his proposal (Trans. Linn. 

 Soc. N. S. W. vol. ix. pt. i. p. 44, 1SS4) to subdivide the Selachoidei, in accord- 

 ance with the great diversities in their skeletal anatomy, into the two suborders 

 of the ValcBOSclachii (Xofulanidcs) and Kcosclacliii. 



It is worthy of remark that the spii-acular gill of these Sharks, although a 



