FRESHWATER MOLLUSKS. 11 



milk white, its blue-black eyes showing conspicuously at the base of 

 each tentacle. It has two characteristics almost peculiar to itself ; 

 ouo is, two almost crescent-shaped projections on either side of 

 the front of the foot, reminding one somewhat of the feet of birds in 

 Noah's Arks. The other is its branchia, or breathing apparatus, 

 which is somewhat like a feather, and is sometimes protruded outside 

 the shell above the head, protected by a third tentacle, which curls 

 about as a sentry to see that the coast is clear. 



Both the species are small. 



5th and last. Nerititia of which we have but one British species. 



It is not unlike in shape to Nerita, the little yellow shell that is so 

 common on our coasts on rocks at low tide. It may be found on 

 stones or on the gravelly bottom of rivers and streams, bat not, I fear 

 in the still waters of the canal. 



Before closing my paper, I must make a few remarks on the order 

 generally. 



Some of them are hermaphrodites ; i.e. contain the male and female 

 organs in one individual ; others have the sexes distinct. Some bring 

 forth their young alive, without any distinct eggs ; others liko Paliidina 

 hatch their eggs within their body. They have no distinct organ of 

 smelling as yet discovered, although they undoubtedly, for the most 

 part, possess that sense. The organs of hearing in some of them are 

 very curious. They consist of two round cells containing fluid and 

 crystalline particles, called otolites or ear stones, which, by means of 

 minute hairs or ciliffi, spin round and round at a tremendous pace, and 

 will continue to do so for some hours after they have been removed 

 from the animal, when the cilisa stiifen and di'op off and all motion 

 ceases. The best creature for obtaining these otolites from is Paludina, 

 the handsome purple-banded shell I have spoken of. 



The organs of respiration vary ; some have branchia, or gills, some 

 have lungs, and some both. The tongue is, in all the Gastropoda, an 

 organ for the attrition of its food. It could not, one would think, con- 

 vey any sense of taste to the so-called brain, as it is of a silicious or 

 flinty nature. It forms a beautiful object for the microscope. 



There are many other objects of exceeding interest connected with 

 these animals ; their microscopic appearance in the early stages of de- 

 velopment, their distribution throughout the globe, theories connected 

 with origin and range of species, which I hope, with the leave of the 

 president and committee, to discuss on a future occasion in a paper on 

 the laud mollusks. 



I can only conclude by recommending any who desire to become 

 further acquainted with these interesting creatiires to search for them- 

 selves, and to set up an aquarium, by which alone any real knowledge 

 of their habits and other points of interest connected with tliem can 

 be acquired ; and I shall be most happy to render any assistance in the 

 way of instraction or advice to those desirous of doing so, and also to 

 lend and recommend books on the subject. 



