144 ANATOMY AND LIFE HISTORY OF MOLLUSCA. 



no way less elaborate than that which is used for the purposes of 

 vision amongst other animals. A vertical section through the 

 tegmentum and the centre of one of the eyes, shows structures 

 in the following order : — 1. A calcareous cornea. 2. An iris. 

 3. A lens. 4. A retina, with rods, to the inner surface of which 

 are attached the optic nerves. The most of these tissues or 

 organs are included within a pigmented capsule, the opening of 

 which exteriorly forms the iris. As far, therefore, as his 

 observations on the Chitonidse extended, the eyes are most 

 complete and elaborate. No doubt further observations are 

 necessary, and will not be long in forthcoming, but what we have 

 is amply sufficient to prove that nothing is wanting to these 

 organs to entitle them to be regarded as eyes of the most 

 complete kind. 



Now, though I am unable to furnish sections of so complete 

 and satisfactory a kind as those given by the Linacre Professor of 

 Anatomy, not having either the technical skill or the technical 

 apparatus for the purpose ; yet still I have been able to satisfy 

 myself that I have been dealing with the same organs in many 

 Australian shells which he has found in the Chitonidse. I find 

 first of all that these little glassy structures, when examined with 

 higher powers under the microscope, show the curtain of 

 pigment-cells in the centre forming a ring which we call an iris, 

 though probably it may not be the homologue of what is usually 

 understood by that term. Furthermore in Patella tramoserica, 

 within the small crystalline capsule, there is a lens and a retina, 

 with a layer of rods or cells containing the ultimate fibrils of 

 nervous tissue. Behind this the strands of the optic nerve spread 

 out, so that the eyes are not of the vertebrate type, but of the 

 form usual amongst Mollusca. At the base of the capsules, 

 directed somewhat obliquely into the substance of the tegmentum, 

 is a nerve-channel, which appears in the form of an extremely 

 delicate tube or sheath. These sheaths are so closely associated 

 together that they give a fibrous appearance to the stratum in 

 which they occur, and their oblique direction is crossed at right 

 angles by other fibres. Or again this stratum of nerve-fibres 

 has them arranged at almost every angle, so that they give a 

 hackled appearance to the stratum. The fibres terminate at a 

 horizontal line of division which separates the crowded nerve-fibres 

 from the nacreous layer. In this is a mass of nervous tissue which 

 I shall distinguish as Neurospongium.* I find it forming large 

 ganglia connected with the eyes and sense-organs in many shells. 



* This term has been proposed by Mr. Sydney Hickson in his paper on 

 the " Optic Tract of Insects." (Quarterly Journal of Microscopical 

 Science, Vol. xxv., p. 219.) He says " The opticon itself consists of a 

 very fine granular matrix, traversed throughout by a fine mesh-work of 



