Ncrvo2is System and Sefiscs. 529 



^filaments are sometimes retractile, and sometimes not : many 

 species do not possess them ; but, when they are present, they 

 become the creature's chief ornament. The shell of the 

 Haliotis, for example, if we except the splendid iridescence 

 of its interior, is sufficiently plain and vulgar ; but behold it 

 borne along by the living tenant, its garniture all displayed 

 and vermicular in the smooth and crystal water, and it 

 moves wonder and admiration, (^g. 106.) The Trochi, and 



many other univalves, afford equally fine illustrations. Among 

 bivalves a fringe of these filaments is very general. In the 

 genera which have the cloak completely open, as the oysters, 

 or the sea and fresh- water muscles, the filaments fringe it all 

 round ; and in those in which the cloak opens by a tube only, 

 these appendices are attached to the circumference of its 

 orifice. Such is the case in the genera Venu5, Cardium, &c. 



Now, these tentacula and filaments are exquisitely sensible, 

 and, in all likelihood, convey impressions of a more distinct 

 character than the general surface. When the Mollusca walk 

 abroad, these organs are all extended to the utmost, and in 

 perpetual motion ; sentinels alive to every thing around, warn- 

 ing against foe or danger, and w^atchful of prey. By means 

 of them they likewise feel their way, and ascertain the nature 

 of the ground they traverse, as it seems evident from the man- 

 ner in which they use them ; but to this purpose the proper 

 tentacula are never applied, at least when they carry the eyes 

 on their tips ; and they appear to be organs of some other 

 sense. If removed, the snail creeps on as if it were unmu^ 

 tilated ; and there are tribes, the Doris is one, in which their 

 position is such, that they cannot possibly be applied to 

 objects either in front or around them. 



2. Taste, — Swammerdam found, by experiment, that snails 

 have " a nice appetite and taste ; " and it seems necessary to 

 suppose the existence of this sense in all Mollusca, for they 

 select particular articles of food in preference to others ; and 

 we know no other sense which is fitted to regulate the choice. 



Vol. IV. — No. 22. m m 



