.
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