Figure 2. Median section of the same. To the right a more diagrammatic figure. The nerves are numbered:-- [BR1, BR2, BR3, BR4 branches of X forking over the second to the fifth gillslit.] cb., cerebellum. h.s.c., horizontal semi-circular canal of ear, exposed by the slicing down of the otic mass. [LAT., lateral-line branch of X.] m.o., medulla oblongata. oph., ophthalmic nerve (V.1+VII.1). op.l., optic lobe. pit., pituitary body. pr.c., prosencephalon (cerebral hemisphere). rh., olfactory lobe (rhinencephalon). r.t., -its- restiform tracts [of medulla]. -st-. [S.P.G.], stalk of the pineal gland. th., thalamencephalon. th.c., thalamencephalon. -ut., the utriculus, seen through the semi-transparent cartilage-. Vid., the Vidian branch of VII. [Visc., visceral branch of X.] Figure 3. Diagram of the ear of a fish. The structure of this is easily made out by clearing otic capsule and cutting slices of the cartilage in the Dog-Fish (e.g., Figure 1, h.s.c.). amp., their ampullae. a.v.c., p.v.c., h.c., anterior, posterior, horizontal canal respectively. [amp., the ampullae.] d.e., the ductus endo-lymphaticus. -sac., the sacculus; c., a small outgrowth of the latter, corresponding to the rabbit's cochlea-. -ut., the utriculus-. Figure 4. The cranium and branchial bars of a Dog-Fish. The groove in the otic capsule connects the orbital and anterior cardinal sinuses. A.C.S., position of the anterior cardinal sinus (dotted outline). c., the vertebral centra. c.b., the cerato-branchial. c.h., the cerato-hyal. e.b., epi-branchial. ex.b., extra-branchial. h.M., the hyo-mandibular. i.n.p., inter-neural plate. M.C., Meckel's (lower jaw) bar. Na.C., the nasal capsule. n.p., neural plate. n.s., neural spine. Ot.C., the otic capsule. ph.b., the pharyngo-branchial. P.pt., the palato-pterygoid bar (upper jaw bar). p.s., pre-spiracular ligament, containing a cartilaginous nodule. r., rib. sp., the position of the spiracle. Figure 5. Diagrams of a vertebral centrum.-- For reference letters, see text (Section 9). {No Figure 6, in First Edition.} [Figure 6. Diagram for comparison with Figure III., Sheet 9.] {Illustration: Diagram Sheet 18.} Sheet 19. Figure 1. Amphioxus, seen from the right side. a----b shows the natural size. The animal is supposed to be clarified, and mounted in some highly refracting medium, so that it is practically transparent; I., II., III., and etc., refer to the section figured on Sheet 20. Figure 2. Amphioxus, General Dissection. (Slightly altered from a figure by Professor E. R. Lankester.) The ventral atrial wall is removed. The pharynx cut away from the dorsal body-wall, and with the true ventral body-wall turned over to the (animal's) right. The arrow a., a., passes through anus to intestine; b., b., is thrust through the atrial pore to the atrial cavity. Note coe., the body cavity.