146 ANATOMY AND LIFE HISTORY OF MOLLUSCA. 



referred to ; but generally it forms a cloud of pigment which soon 

 merges into the highly refractive nerve-substance which forms the 

 ganglion supplying all these sense-organs. 



In one section of Patella the nerve-fibres form a dark cloud of 

 pigment at the base of the tubes, but swept to one side like smoke 

 from a chimney. In another section there were alternate dark 

 and light bands something like rain-clouds on the edge of the 

 horizon at sea. As a matter of fact, it would be difficult to 

 describe the appearances presented in each individual case without 

 entering endlessly into detail, and therefore I must content myself 

 with saying generally that nerve-fibres can be seen entering or 

 emerging from the tubes which subtend the eye-capsules and 

 diffusing themselves more or less thickly through the substance of 

 the nacreous layer. Here they may be followed until they unite 

 in a darker cloud of nerve-tissue and form a ganglion of what I 

 term neurospongium. 



These ganglia in the shell substance are of such peculiar structure 

 that most probably the differences from neurospohgium in the 

 opticon of insects will hereafter be found more numerous than the 

 resemblances. It seems as if the substance of the nacreous shell 

 was perforated by innumerable foramina, which concentrate from 

 all directions in one point. These are filled with strands of nerve 

 tissue of the usual character ; but these strands are often marked 

 with pigment which gradually increases in quantity so that the 

 centre of the ganglion is rendered almost dark by it, though it is 

 relieved by the innumerable round refractive bodies such as are 

 usually seen associated with nerve substance. I shall have occasion 

 to speak of these ganglia in the substance of the shell further on. 

 As to the dark pigment, the observations already referred to from 

 Mr. Patten will serve as an explanation.* 



At the base of some of the membranous tubes leading up to the 

 micropores, there are certain dark bodies, very much like the 

 pigment cells of eyes. They are round, and they seem constructed 

 to fill up the ends of the tube, and moreover they have a central 

 nucleus or a pupil-like dot. They seem like eyes ; but the question 

 will naturally arise, what purpose could eyes serve thus deep in 

 the substance of the shell ; for they are fully a millimetre or more 

 beneath its surface, which relatively to their size, is a deep burial. 

 Yet as the tube is open they may be deep-seated eyes, and this 

 I am inclined to think is their function. 



It should be stated here that in the case of Patella and some 

 other shells, the membranous tubes here referred to, even when 

 they have not these deep-seated eyes, bear so strong a resemblance 

 to the cones of an insect's eye, that it is not difficult to suppose 



* " The Eyes of Mollusks and Arthropods," Mittheilungen aus der 

 Zoologischen Station zu Neapel, Vol. vi., Berlin, 1866, pi. 29, fig. 19. 



