372 ME, J. H, VANSTONE ON SOME POINTS IN THE 



the exact nature of this layer, as it and the neighbouring tissues 

 were in an insufficiently preserved state *. 



The thickened cuticle in the stomach of Crepidula, according: 

 to Haller t, is perfectly homogeneous, without lamellse, cells, oi 

 nuclei, and in these respects it differs from that of Melongena. 

 In certain Cestodes (e. g. Tcenia lineatd) there is present a 

 lacunar tissue beneath the cuticle such as we have seen in 

 Melongena, and its resemblance to the connective tissue of a 

 mollusk has been shown by Grriesbach %. The lacunae are either 

 empty or filled with granular contents, and both these conditions- 

 are met with in Melongena. 



The structures which I have herein described are situated m 

 the mesenteron, and are derivative of that portion of the alimen- 

 tary canal, and they must therefore be distinguished from the 

 plates and hooks met with in many other Grasteropods (e. g. 

 Aplysia §), in which they occur in the parts derived from the 

 stomodeum. These stomodeal plates, moreover, are chitinous in 

 nature. Barrois compares the " sagitta " of the Lamellibranch 

 and its representatives above the Gasteropoda with th.e chitinous 

 lining (" Trichter ") of the gut of Insects and other Arthropods ; 

 but it seems doubtful whether he is right in doing this, seeing 

 that while the " Trichter" is confined to the fore and hind gut, 

 the mesenteron (" chylific stomach " of the Insects) is devoid of 

 a cuticular lining and derivative of the hypoblastic midgut. The 

 structures herein described are unlike the cuticular derivatives 

 of the stomodeum and proctodeum, among the Invertebrates j. 

 developed internally to the alimentary epithelium, which may 

 itself be converted into a cuticle-like covering where they occur 

 {ex. sh., fig. 7). So far as they may be compared to any known 

 accessories of tlie alimentary system of other Mollusca, they 

 suggest a kinship to the cartilaginoid supports of the odonto- 

 phore. 



For the material upon which this investigation has been per- 

 formed I am indebted to the kindness of Mr. M. F. Woodward,, 

 and to Prof. G. B. Howes for many valuable suggestions. 



* It may be incidentally remarked that the structure of the plates has a 

 superficial resemblance to masses of coagulated blood in the surrounding organs.. 



t Haller, Morph. Jahr. xTiii. p. 602, and Taf. 18. figs. 84-86. 



X Griesbaeh, Arch, fiir mikr. Anat. xxii. 1883. 



§ In the case of the chitin-bearing segment in Aplysia, there is wanting proof 

 of its derivation from the stomodeum. 



