190 THE MICROSCOPE AND IT8 REVELATIONS. 



he may find convenient. A magnifying power of about 120 diameters is 

 amply sufficient to afford a general view of this spectacle; but a much 

 greater amplification is needed to bring into view the peculiar mode in 

 which the stroke of each cilium is made. Few spectacles are more 

 striking to the unprepared mind, than the exhibition of such wonderful 

 activity as will then become apparent, in a body which to all ordinary ob- 

 servation is so inert. This activity serves a double purpose; for it not only 

 drives a continual current of water over the surface of the gills them- 

 selves, so as to effect the aeration of the blood, but also directs a portion 

 of this current (as in the Timicata, 555) to the mouth, so as to supply 

 the digestive apparatus with the aliment afforded by the Diatomacece, 

 Infusoria, etc., which it carries-in with it. 



" 587. Organs of Sense of Mollusks. Some of the minuter and more 

 rudimentary forms of the special organs of sight, hearing, and touch, 

 which the Molluscous series presents, are very interesting objects of Mi- 

 croscopic examination. Thus, just within the margin of each valve of 

 Pecten, we see (when we observe the animal in its living state, under 

 water) a row of minute circular points of great brilliancy, each surrounded 

 by a dark ring; these are the eyes, with which this creature is provided, 

 and by which its peculiarly-active movements are directed. Each of them, 

 when their structure is carefully examined, is found to be protected by a 

 sclerotic coat with a transparent cornea in front; and to possess a colored 

 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 pro- 

 ceeding from an optic nerve which passes to each eye from the trunk that 

 runs along the margin of the mantle. 1 Eyes of still higher organization 

 are borne upon the head of most Gasteropod Mollusks, 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 tentacles 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 eifected 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 in 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 Mollusks have distinct organs of 

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

 (Fig. 403, K, x), attached to some part of the nervous collar that sur- 

 rounds 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 sacculi, 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 spec- 

 tacle," as it was truly designated by its discoverer Siebold, may be brought 

 into view without any dissection, by submitting the head of any small 



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

 Sci.," Vol. xx., N.S. (1880), p. 443. 



