INTRODUCTION. 19 



shells, and other submarine substances : they are more or 

 less gelatinous. The only powers of motion which they 

 possess seem to be that of contracting and dilating them- 

 selves alternately; by which means they are enabled 

 to eject, with considerable force, the water which they 

 imbibe. This animal inhabits the Pholas, Solen, some 

 species of the Mya, Mactra, and other bivalves. 



Tethys. The body is detached, rather oblong, fleshy, 

 without peduncles : the mouth is furnished with a termi- 

 nal cylindrical proboscis, under an expanded membrane 

 or lip; apertures two, on the left side of the neck. The 

 Tethys inhabits a great proportion of bivalve shells, as, 

 many species of Tellina, Cardium, Mactra, Venus, Ostrea, 

 and others. 



Limax. The body is oblong, creeping, with a fleshy 

 kind of shield above, and a longitudinal flat disc beneath : 

 aperture placed on the right side within the shield : feel- 

 ers four, situated above the mouth, with an eye at the 

 top of each of the larger ones. The animals belonging to 

 this genus inhabit the turbinated univalve shells; but it 

 appears that all the animals which inhabit these shells do 

 not exactly correspond with the above generic characters. 



Spio. The body projecting from a tube, jointed and 

 furnished with dorsal fibres ; peduncles or feet rough with 

 bristles, and placed towards the back; feelers two, long, 

 simple ; eyes two, long. This animal inhabits some spe- 

 cies of Sabella. 



Amphitrite. Body projecting from a tube, and annu- 

 late; peduncles or feet small, numerous, with lateral fas- 

 ciculi, and branchiae; feelers two, approximate, feather- 

 ed ; no 'eyes. The Amphitrite inhabits some species of 

 Sabella and Serpula. 



Terebella. Body oblong, creeping, naked, furnished 

 with lateral fasciculi or tufts, andbranchias; mouth placed 



