PBBSENCE OP EYES IN SHELLS OP OERTAIN CHITONIDJl. 25 
PLATE Yl. 
Fig. 1. — Sketch of a lateral view of a specimen of Acanthopleura 
spiniger. a- Small area on thè side of thè tegmentum of one of thè middle 
shells, which is shown more highlj magnified in Eig. 2. 
PiG. 2. — The area indicated in. thè foregoing figure eniarged 65 diameters. 
a a. Prominent rounded tubercles on thè shell surface. b b. Pore-hillocks, 
each witli a megalopore visible at its summit, d d. Eyes. 
Fig. 3. — Three of thè pore-hillocks of Acanthopleura spiniger shown 
in thè preceding figure more highly magnified. a. Megalopore. b. Micro- 
pore. 
• Fig. 4. — View of thè surface lajer of thè soft tissues of a decalcified middle 
shell of a species of Acanthopleura from China, viewed hy transmitted light, 
showing thè grouping of thè micreesthetes and megalmsthetes in relation with 
an eye. The cuticula of thè tegmentum is seen to he finely fibrous. n. Chan- 
nel, containing thè optic nerve. l. Lens of thè eye. eh. Pigmented eye- 
capsule, forming an iris-like border around thè lens. b. Tips of micraesthetes, 
which pierce thè iris and terminate at thè free surface of thè shell around thè 
margin of thè cornea, a a a. Macraesthetes in chambers, hollowed out in 
thè tissue and continued in thè direction of thè margin of thè shell which 
adjoins thè girdle into nerve canals. 
Fig. 5. — A single eye of Schizochitou decalcified. l. Lens. eh, Pigmented 
eye-capsule, continued inwards to form an iris, b b. Slit-like apertures in 
thè iris, giving passage to thè branches given off by thè optic nerve to thè 
ocular micraesthetes. s, Cleft in thè outer wall of thè channel for thè optic 
nerve. 
Fig. 6. — Schemalic representatiou of thè structure of thè soft and some 
of thè hard parts in thè tegmentum of a shell of Acanthopleura spiniger, 
as seen in a section vertical to thè surface and with thè margin of thè shell 
bordering on thè girdle Ijing in thè direction of thè left side of thè drawing. 
f. Calcareous cornea. h. Iris. g. Lens. k. Pigmented capsule of eye. 
n. Optic nerve. r. Rods of retina. 7^ . Branches of thè optic nerve, per- 
forating thè capsule wall and terminating in b' b' b'. Ocular micraesthetes. 
p p, Nerves to macraesthetes. m. Body of macraesthete cut across. o e. Fusi- 
form body of macraesthete entire. a, Obconical termination of macraesthete. 
e. Nerve given off bj macraesthete to micraesthete 6". 
Fig. 7. — Rods of thè retina of Acanthopleura spiniger, viewed from 
above in a horizontal section of thè eye. 
Fig. 8. — Sketch of a specimen of Acanthopleura plceus, viewed from 
thè side, of naturai size. x. Area on thè side of thè tegmentum of one of thè 
middle shells, which is seen highly magnified in Fig. 9. 
Fig. 9. — The area indicated in Fig. 8, eniarged 26 diameters. The lateral 
