TRANSACTIONS OF SECTION D. 



781 



f OUCH ou.J^.....vv,. ^..^O^ .l...j.....^.-...v,.^., ....Vl ....^.i^^w.v v>.t, ...wvi 



r pores are dispo.sed on the surface of the tegmentum with i 

 .nilarity in dilierent genera of Cliitonidie, in many cases in very 



more or less exact 

 detinite lines and 



Tlie network terminates at the surface of the tegmentum all over in a series of 

 (-li)n''ate cylindrical organs of toucli (* megalsesthetes "), the phig-lilve ends of which 

 nre somewhat dice-box shaped, and can be protruded beyond the level of tiie 

 tegmental surface from a series of pores (' megalopores") by wliich this surface is 

 envered. These larger organs of toucli give oil" from their .«ides tine branches of 

 soft tissue, which i)a.«s vertically to the surface of the tegmentum and terminate 

 there in minute plug-like organs, like the larger ones but much smaller ('micrffis- 

 tlietes'), and which are protrusible from a series of smaller pores (' micropores ') in 

 t!ie shell substance. Tliese megal;T^.stlietes and micrfesthetes and their correspond- 



iiisf 



regularity 



patterns. 



The eyes are connected with the same network of soft tissue as the touch- 

 nvirans, and are apparently to be regarded as having arisen in (levelopnicnt as specinJ 

 moditications of tliem. The .soft structures of ciieh eye lie in a more or less pear- 

 shaped chamb.;r excavated in the sub.-taucj of the tegmentum. Tlie stalk of the 

 pear, wliich forms tlie canal for the jiassage of the optic nerve, is directed always 

 towards the full margin of the tegmentum, and hero its wall is jiierced by a circuh'ir 

 iiperture which is covered by the cornea. The cornea is calcareous, resisting th« 

 action of strong boiling caustic alkalis, but collapsing at once when treated with 

 acids. In sections it is seen to be composed of a series of concentric lamelho. Its 

 •substauce is continuous with the general calcareous substance of the tegmentum at 

 its margins. 



The pear-shaped cavitj' of the eye formed by the shell substance is lined 

 bv a dark-brown pigmented membrane, of a still' and apparently somewhat 

 ciiitinous texture. This membrane exactly follows the shape of the cavity, but by 

 projecting inwards beyond the margin at the cornea all round, forms an' iris of a 

 less diameter than the latter. A perfectly hyaline, strongly biconvex h'us is placed 

 behind the iris aperture. It is composed of soft tissue, and dissolves in st rong acetic 

 acid. The optic nerve at some distance from the retina is a compact strand, but 

 liefore reaching the latter lias its numerous tine fibres separated and loose. The 

 retina is composed of a single la^'er of rather short but extremely distinct nucleated 

 rods of roughly hexagonal section, witii their free ends presented to the light. Im- 

 mediately behind them is a dense mass of nerve fibres with numerous nuclei and 

 nerve cells interspersed. The retina is on the type of that of Helix, and not, as 

 might have been supposed, on that of the dorsal eyes of Onchidium. 



A large part of the peripheral fibres of the optic nerve do not jjass to the retina, 

 but pass outside the eye chamber by a series of apertures in the choroid round tlie 

 iris margin, and end at the shell surface in a zone of micrresthetes encircling the eye. 

 The micraesthetes are identical in structure with the smaller touch organs already 

 described as appended to oilsets of tlie megahesthetes all over the shell. In giving 

 dlf nerves to a series of such small organs, the eye thus corresponds exactly in 

 s-tructure to the megaltesthetes, and its homogeuv with them is thereby clearly 

 indicated. The arrangement of the eyes varies much in the different genera. 



In Schizochiton incisus the eyes are restricted to single rows traversing the linos 

 separating the lateral area? from the areoe centrales, and corresponding in position 

 M'ith the incisuroe laterales and courses of tiie principal nerve.s. There are six rows of 

 eyes, with six marginal slits on the anterior shell and six on the posterior, and a 

 single pair on each of the intermediate shells, twenty-ic • rows in all, with an 

 average of about fifteen eyes in each, or in all .300 eyes. 



In Acantkopleura spiiiigfr the eyes are irregularly scattered around tiie base.s 

 of tlie tubercles with which the surface of the tegmentum is covered, and are confined 

 in the specimens examined to the region of the margins of the shells adjoining tbe 

 mantle. The surface of the older regions of the tegmentum seems in this species 

 especially liable to flake ofl", carrying the eyes with it, and it will probably be found 

 wlien series of examples of various eyes are examined that the eyes are originally 

 more widely extended over the shell surfaces. In Curephium aculeatum the eyes 

 are very small, with corneas oval in outline, the long axis of the oval being directed 

 vertically to the shell margin. They are never placed on the tubercles, with rowa 



