{230}[Illustration: FIG. 118.—Diagram explaining the possible relations of Rotifers. A, _Pilidium_; B, hypothetical Rotifer modified from _Asplanchnopus_; C, a Ploimal Rotifer; D, _Trochosphaera aequatorialis_ (modified from Semper, the extension of the ovary into the posterior ventral quadrant being omitted); E, Mollusc larva (Veliger); F, Trochophore larva of Annelid. _a_, Anus; _ap_, apical organ; _at_, median antenna (near which, in B, is a black spot, the brain); _bl_, bladder (receiving the ramified kidney in B, C, and D); _br_, brain; _f_, foot; _fg_, cement-glands, replacing apical organ; _g_, ovary; _k_, kidney; _m_, mouth; _n_, supra-oesophageal ganglion; _nap_, nerve of apical organ; _nr_, nerve-ring in section; _pot_, praeoral portion of trochus; _s.g_, shell-gland; _s.n_, sub-oesophageal ganglion; _t_, trochus or ciliary wreath; _tt_, posterior ciliated ring; _v_, velum, or expanded praeoral part of trochus.] The sides are produced on either side into lappets, which we do not take into account. A cup-shaped depression at the apical pole is lined by sense-cells, bearing long cilia which are probably sensory. A ring of nerve-cells passes within the ciliated rim of the hemisphere, and the stomach is a blind sac. If we compare this organism with a Rotifer, we find that the wreath corresponds in both, the funnel of the disc in such forms as Flosculariidae and _Microcodon_ leading to the mouth of _Pilidium_, while the gut is blind in Asplanchnidae and in some of the highly developed Seisonidae. The circular nerve-ring of _Pilidium_ is in many Rotifers only represented by its anterior part, the brain; though in Bdelloids a sub-oesophageal ganglion completes the ring. This leaves a difficulty with regard to the apical sense organ; but it is easy to understand that an organ of sensation should become an organ of fixation. In this case the foot with its glands would correspond to the sense organ of the Trochophore larva; and it retains its primitive ciliated character in the larvae and males of many Rotifera, and the adult female of _Pterodina_ and _Callidina tetraodon_. Embryology tells {231}us that the anus of Rotifers cannot be homologous with that of Annelids, etc., for it is formed outside the area of the blastopore: it is an independent formation, probably due to the coalescence of the originally blind intestine at its extremity with the earlier genito-urinary cloaca. On this view we must change the orientation of the Rotifer, and place it, like a Cuttlefish, mouth downwards: for "anterior and posterior" we must substitute _oral_ (or _basal_) and _apical_; for "dorsal" and "ventral" we must use _anterior_ and _posterior_; while "right" and "left" are unchanged. And this correctly expresses the actual space-relations in those Ploima like _Rattulus_ that swim with their disc in contact with the organic débris on which they feed, with the foot turned outwards and backwards. As these views are now