TRANSACTIONS OF SECTION D. 781 



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

 elongate cylindrical organs of touch (' megalaesthetes '), the plug-like ends of which 

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

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

 covered. The3e larger organs of touch give off from their sides fine branches of 

 soft tissue, which pass vertically to the surface of the tegmentum and terminate 

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

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

 the shell substance. These megalsesthetes and micraesthetes and their correspond- 

 ing pores are disposed on the surface of the tegmentum with more or less exact 

 regularity in different genera of Chitonidae, in many cases in very definite lines and 

 patterns. 



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

 organs, and are apparently to be regarded as having arisen in development as special 

 modifications of them. The soft structures of each eye lie in a more or less pear- 

 shaped chamber excavated in the substance of the tegmentum. The stalk of the 

 pear, which forms the canal for the passage of the optic nerve, is directed always, 

 towards the full margin of the tegmentum, and here its wall is pierced by a circular 

 aperture which is covered by the cornea. The cornea is calcareous, resisting the 

 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 lamellae. Its 

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

 its margins. 



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

 by a dark-brown pigmented membrane, of a stiff and apparently somewhat 

 chitinous 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 lens is placed 

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

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

 before reaching the latter has its numerous fine fibres separated and loose. The 

 retina is composed of a single layer of rather short but extremely distinct nucleated 

 rods of roughly hexagonal section, with 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 pass to the retina, 

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

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

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

 described as appended to offsets of the megalaesthetes all over the shell. In giving 

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

 structure to the megalaesthetes, and its homogeny 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 lines; 

 separating the lateral areae from the areae centrales, and corresponding in position 

 with the incisurae laterales and courses of the principal nerves. 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-four rows in all, with an 

 average of about fifteen eyes in each, or in all 360 eyes. 



In Acanthopleura spiniger the eyes are irregularly scattered around the bases 

 of the 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 the 

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

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

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

 more widely extended over the shell surfaces. In Corephium 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 rows 



