SENSE-ORGANS OF MOLLUSCA 941 



cornea in front, and to possess a coloured iris (having a pupil) that 

 is continuous with a layer of pigment lining the sclerotic, a crystalline 

 lens and vitreous body, and a retinal expansion proceeding from an 

 optic nerve which passes to each eye from the trunk that runs along 

 the margin of the mantle. 1 Professor H. N. Moseley made the 

 interesting discovery that many of the Chitonidce are provided with 

 a large number of minute eyes on the exposed areas of the outer 

 surfaces of their shells ; as the fibres of the optic nerve are directed 

 to the rods from behind these eyes are of the ordinary invertebrate 

 type, and differ therein from the just mentioned eyes of Pecten, or 

 those which are found on the back of Onchidium, which resemble 

 the vertebrate retina in having the optic fibres inserted into the front 

 aspect of the layer of rods. 2 Eyes* of still higher organisation are 

 borne upon the head of most Gastropod molluscs, generally at the 

 base of one of the pairs of tentacles, but sometimes, as in the Snail 

 and Slug, at the points of these organs. In the latter case the ten- 

 tacles are furnished with a very peculiar provision for the protection 

 of the eyes ; for when the extremity of either of them is touched it 

 is drawn back into the basal part of the organ, much as the finger of 

 a glove may be pushed back into the palm. The retraction of the 

 tentacle is accomplished by a strong muscular band, which arises 

 within the head and proceeds to the extremity of the tentacles ; 

 whilst its protrusion is effected by the agency of the circular bands 

 with which the tubular wall of the tentacle is itself furnished, the 

 inverted portion being (as it were) squeezed out by the contraction 

 of the lower part into which it has been drawn back. The structure 

 of the eyes and the curious provision just described may easily be 

 examined by snipping off one of the eye-bearing tentacles with a pair 

 of scissors. None but the Cephalopod molluscs have distinct organs 

 of hearing ; but rudiments of such organs may be found in most 

 Gastropods (fig. 711, K, x), attached to some part of the nervous 

 collar that surrounds the oesophagus, and even in many bivalves, in 

 connection with the nervous ganglion imbedded in the base of the 

 foot. These ' auditory vesicles,' as they are termed, are minute sac- 

 culi, each of which contains a fluid, wherein are suspended a number 

 of minute calcareous particles (named otoliths, or ear- stones), which 

 are kept in a state of continual movement by the action of cilia 

 lining the vesicles. This 'wonderful spectacle,' as it was truly 

 designated by its discoverer Siebold, may be brought into view 

 without any dissection by submitting the head of any small and not 

 very thick-skinned Gastropod, or the young of the larger forms, to 

 gentle compression under the microscope and transmitting a strong 

 light through it. The very early appearance of the auditory vesicles 

 in the embryo Gastropod has been already alluded to. Those who 

 have the opportunity of examining young specimens of the common 

 Pecten will find it extremely interesting to watch the action of the 



1 See Mr. S. J. Hickson on ' The Eye of Pecten ' in Quart. Journ. Microsc. Sci. 

 vol. xx. n.s. 1880, p. 448, and K. E. Schreiner, ' Die Augen bei Pecten und Lima,' 

 Bergens Mus. Aarbog, 1896, no. 1. 



2 See Professor Moseley ' On the Presence of Eyes in the Shells of certain Chitonidse 

 and on the Structure of these Organs,' in Quart. Journ. Microsc. Sci. xxv. p. 37. 



