PRESENCE OE EYES IN SHELLS OF CERTAIN CHITONID2E. 11 
of trausverse ring lines (PI. VI^ tìg. 6, a) as if composed of a 
series of superposed discs. Towards tlie base of thè knob 
these lines instead of being simply transverse become bent 
towards thè body of thè megalaesthete as if thè knob were there 
composed of a series of cup-like layers. Between these trans- 
verse lines thè tissue of thè knob is dotted with very fine 
granules. The knobs of thè megalsesthetes appear to be capable 
of protrusion from thè mouths of their pores and retraction^ as 
many were found protruded in spirit specimens^ To thè organs 
contained within thè micropores I shall give thè name micrses- 
thetes. Van Bemmelen’s figure, above referred to, shows four 
micrsesthetes as given off from thè summit of thè body of a 
megalaesthete of Chiton marginatus. 
In PI. VI, fig". 6, areshown similar micraesthetes supplied by 
small strands given off from thè megalaesthetes in Acantho- 
pleura spiniger. The micraesthetes are small, knob-like 
bodies, exactly corresponding in structure to thè knobs of thè 
macraesthetes. They are similarly obconical in form, and 
exhibit exactly similar concentric and transverse ring-marks 
(PI. V, fig. 8, b), and are obviously homologous organs. They 
are thè terminations of fine strands of tissue, which in 
Acanthopleura spiniger and thè genus Chiton appear to 
be given off only from thè sides of thè megalaesthetes and from 
thè optic nerves, but in Corephium aculeatum (PI. V, 
fig. 8, b) spring independently directly from thè large vertical 
branches of thè main network. 
General Position and External Appearance of 
thè Eyes. 
The eyes in thè Chitonidae are entirely restricted to thè 
outer surfaces of thè shells on their exposed areas — thè surfaces 
of thè tegmenta. They never occur on thè laminae of insertion, 
thè articulameta, nor on thè girdle or zone of thè mantle which 
is occupied, as is well known, by various calcareous structures, 
some of which bave been carefully investigated by Keincke.^ 
* “Beitràge zur Bildungsgescliichte der Stacbeln, &c., im Mantel rande 
der Cliitonen,” ‘ Zeilsch. fiir wiss. Zool.,’ Bd. xviii, S. 305. 
