130 ANATOMY AND LIFE HISTORY OF MOLLUSCA. 



substance of the shell, which follow the line of separation between 

 the area centralis and the areai laterales of the tegmentum. These 

 narrow tracts are permeated by numerous longitudinal canals, 

 which lodge each a specially large stem of soft tissue and nerves, 

 which ramifies in the substance of the tegmentum. Corresponding 

 with this tract, on the under surface of the shell, are a series of 

 minute openings leading into it, through which further strands 

 of soft tissue, possibly mostly nervous, pass from the surface of 

 the shell-bed into the shell, to give the general network of soft 

 tissue. In the anterior and posterior shells there are usually 

 a considerable number of such marginal slits, each with a 

 corresponding tubular tract and ramifying strands of soft tissues. 



The network of soft tissues contained in the canals within the 

 tegmentum ramifies towards the shell-surface and terminates there 

 either in eyes or in peculiar elongate bodies, which, apparently, 

 are organs of touch. These latter are long, somewhat sausage- 

 shaped bodies, which terminate at their free extremity in dice-box 

 shaped plugs of transparent tissue, which show a somewhat 

 complicated structure. The tegmenta of the shells of most 

 Chitonidse are perforated at the surface by circular apertures or 

 pores of two sizes, arranged in more or less definite patterns with 

 regard to one another, and sometimes with regard to the eyes 

 also. The end plugs of the sense-organs above described, lie in 

 these larger pores. From the sides of the sausage-shaped sense- 

 organs are given off more or less numerous fine strings of soft 

 tissue which, diverging, pass to the smaller pores above described, 

 and there terminate in very small plugs, just like those of the 

 larger similar organs, but less complex in structure." 



Having disposed of the difficulties with regard to the Chitonidae, 

 he explains the manner in which the soft structures of each eye 

 are located. They lie in a somewhat pear-shaped chamber in the 

 substance of the tegmentum or exposed area of the shell. The 

 portion which would correspond with the stalk of the pear is the 

 canal for the optic nerve, directed towards the free margin of the 

 tegmentum, whence the nerve reaches it. One side of the chamber 

 is pierced by a circular aperture which is covered by a calcareous 

 cornea. This cornea is formed of concentric lamellae, and its 

 substance is continuous at its margins with the shelly tegmentum. 

 The eye-cavity is lined with a dark brown pigment membrane which 

 projects slightly round the margin of the aperture and forms an 

 iris. There is a perfectly transparent hyaline, strongly bi-convex 

 lens composed of soft tissue and of fibrous structure. It dissolves 

 slowly in strong acetic acid. The optic nerve does not perforate 

 the retina, which is composed of a single layer of very short rods 

 with their ends directed towards the light. Some of the fibres of 

 the optic nerve, without proceeding to the retina, pass round 



