270 TOURNAI. OF CONCIIOI.Ol^V, VOI,. 9, NO. 13, JANUARY, I9I2. 



the edge of a wine-glass. This sound resembles an .'J^ohan harp, and 

 being usually heard at dusk or during the night, and the source of the 

 mysterious sounds being frequently unsuspected, has often caused 

 feelings of superstitious dread amongst those ignorant of its origin. 



The sense of Sight is possessed by almost all moUusks, but objec- 

 tive perception is probably not very keen or definite. The eyes are 

 placed at the tips of the upper tentacles, and can be withdrawn for 

 protection within the body by special muscles, but their protrusion 

 is mainly due to blood pressure and the action of the annular tenta- 

 cular muscles. They are of complex structure, but differ from the 

 human eye in so far that the optic nerve does not pierce the retina, 

 and therefore does not form a blind spot. The O/ichidiiiui, a kind 

 of marine slug, has, however, the eyes upon its back, constructed on 

 exactly the same plan as our own. 



The sense of Smell is chiefly exercised by a special development 

 at the tips of the tentacles of a large nerve which arises from a lobe 

 of the brain and terminates in an external layer of olfactory epithelium; 

 this position of the organ being probably an adaptation to and deve- 

 loped by terrestrial life as in the more primitive aquatic moUusks, this 

 sense is located within or at the entrance to the breathing chamber. 



The Alimentary or Nutritive system resembles our own in its 

 general arrangements and in the functions of its organs, but differs 

 in possessing only an upper jaw, which is of the type known as 

 Odontognathous or toothed, and is placed at the entrance of the 

 mouth ; it is composed mainly of chilin, and serves chiefly to hold the 

 food while the sharp teeth rasp off particles, but it is able to bite out the 

 piece if the food be soft. 



The teeth are exceedingly numerous, and arranged like a tessellated 

 pavement in closely packed rows on the upper surface of the radular 

 membrane or tongue on the floor of the mouth cavity. There are 

 about 140 of these rows, with loo or more teeth in each row, or about 

 14,000 teeth in all ; but the identical teeth now seen would not be 

 present during the whole life of the animal, as the membrane to 

 which they are fixed is always growing forward, like a finger-nail, so 

 that the teeth in front are constantly being worn awa\-, while new 

 ones are continually forming behind, which gradually come into use. 

 Some species in this way are said to have twenty or more complete 

 changes of teeth during the life-time of the animal. 



Locomotion is due to the alternate contraction and relaxation of 

 the foot-muscles, and these contractions can be seen as dark waves 

 passing from one end of the animal to the other ; I have invariably 

 counted seven as clearly visible at one time in the present species. 



The speed at which these animals travel is not great, but when in 



