If Salensky's comparison is correct, and there is something to be said for it, the foot or tail of Rotifers is not a post-anal portion of the trunk, but a ventral appendage, and the segmentation which it frequently exhibits is not to be compared with a true segmentation of the trunk. If the Rotifers, as seems not impossible, exhibit crustacean affinities, the 'foot' may perhaps be best compared with the peculiar ventral spine of the Nauplius larva of _Lepas fascicularis_ (vide Chapter on Crustacea) which in the arrangement of its spines and other points also exhibits a kind of segmentation. BIBLIOGRAPHY. (232) F. Cohn. "Ueb. d. Fortpflanzung von Räderthiere." _Zeit. f. wiss. Zool._ Vol. VII. 1856. (233) F. Cohn. "Bemerkungen ü. Räderthiere." _Zeit. f. wiss. Zool._ Vol. IX. 1858, and Vol. XII. 1862. (234) T. H. Huxley. "Lacinularia socialis." _Trans. of the Microscopical Society_, 1853. (235) Fr. Leydig. "Ueb. d. Bau u. d. systematische Stellung d. Räderthiere." _Zeit. f. wiss. Zool._ Vol. VI. 1854. (236) W. Salensky. "Beit. z. Entwick. von Brachionus urceolaris." _Zeit. f. wiss. Zool._ Vol. XXII. 1872. (237) C. Semper. "Zoologische Aphorismen. Trochosphæra æquatorialis." _Zeit. f. wiss. Zool._ Vol. XXII. 1872. CHAPTER IX. MOLLUSCA[99]. [99] The classification of the Mollusca adopted in the present chapter is shewn in the subjoined table: I. ODONTOPHORA. II. LAMELLIBRANCHIATA. 1. Gasteropoda. _a._ Dimya. _a._ Prosobranchiata. _b._ Monomya. _b._ Opisthobranchiata. _c._ Pulmonata. _d._ Heteropoda. 2. Pteropoda. _a._ Gymnosomata. _b._ Thecosomata. 3. Cephalopoda. _a._ Tetrabranchiata. _b._ Dibranchiata. 4. Polyplacophora. 5. Scaphopoda. Although the majority of important developmental features are common to the whole of the Mollusca, yet at the same time many of the subdivisions have well-marked larval types of their own. It will for this reason be convenient in considering the larval characters to deal successively with the different subdivisions, but to take the whole group at once in considering the development of the organs. _Formation of the layers and larval characters._ ODONTOPHORA. Gasteropoda and Pteropoda. There is a very close agreement amongst the Gasteropoda and Pteropoda in the general characters of the larva; but owing to the fact that the eggs of the various species differ immensely as to the amount of food-yolk, considerable differences obtain in the mode of formation of the layers and of the alimentary tract.