250 STILIFER. 



select sand, often small stones, and more frequently the 

 debris of dead shells, belonging to other genera. The 

 TJielidomus, which might be considered as the fresh-water 

 analogue of Phorus, has, I believe, been ascertained to 

 be formed by the larvae of an insect, thus depriving 

 Mr. Swainson of a favourite type among Mollusca. The 

 animals of Phorus are of a dull, opaque-white colour, the 

 eyes large, and black, and the proboscis pinkish. In 

 P. onustus of Reeve, the end of the proboscis is yellow, and 

 the inferior surface pink. The operculum is horny, soft, 

 and flexible, with concentric and radiating fibres covered 

 with ridges, formed by the fibres being elevated, one 

 above the other, in succession. 



Among other peculiarities in the habits of Mollusca, 

 perhaps one of the most striking is the case of Stilifer, 

 a little parasite that lives upon the juices of, and takes 

 up its abode in, the coriaceous integument of Star-fishes. 

 Having, by means of its long, narrow, and slender foot, 

 insinuated itself among the sutures of the armour the 

 Asterias is provided with, it forms a snug nest in the soft 

 parts, where it remains imbedded, with the apex of the 

 spire just protuding. When placed in a watch-glass, 

 under the microscope, I observed that it does not appear 

 to be possessed of the power of locomotion, but that it 

 extrudes its foot to its greatest extent, and makes use of 

 it as an exploring organ, moving it about in all directions. 



