PRESENCE OP EYES IN SHELLS OF CERTAIN CHITONID^E. 45 
of transverse ring lines (PI. VI, fig. 6, a) as if composed of a 
series of superposed discs. Towards the base of the knob 
these lines instead of being simply transverse become bent 
towards the body of the megalaesthete as if the knob were there 
composed of a series of cup-like layers. Between these trans- 
verse lines the tissue of the knob is dotted with very fine 
granules. The knobs of the megalaesthetes appear to be capable 
of protrusion from the mouths of their pores and retraction, as 
many were found protruded in spirit specimens^ To the organs 
contained within the micropores I shall give the name micraes- 
thetes. Van Bemmelen’s figure, above referred to, shows four 
micraesthetes as given off from the summit of the body of a 
megalsesthete of Chiton marginatus. 
In PI. VI, fig. 6, are shown similar micraesthetes supplied by 
small strands given off from the megalaesthetes in Acantho- 
pleura spiniger. The micraesthetes are small, knob-like 
bodies, exactly corresponding in structure to the knobs of the 
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 the terminations of fine strands of tissue, which in 
Acanthopleura spiniger and the genus Chiton appear to 
be given off only from the sides of the megalaesthetes and from 
the optic nerves, but in Corephium aculeatum (PI. V, 
fig. 8, b ) spring independently directly from the large vertical 
branches of the main network. 
General Position and External Appearance of 
the Eyes. 
The eyes in the Chitonidae are entirely restricted to the 
outer surfaces of the shells on their exposed areas — the surfaces 
of the tegmenta. They never occur on the laminae of insertion, 
the articulameta, nor on the girdle or zone of the mantle which 
is occupied, as is well known, by various calcareous structures, 
some of which have been carefully investigated by Beincke. 1 
1 “Beitrage zur Bildungsgeschichte der Stacheln, &c., im Mantel rande 
der Chitonen,” ‘ Zeitsch. fur wiss. Zool.,’ Bd. xviii, S. 305. 
