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 witli 

 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. 



Terehella. — Body oblong, creeping, naked, furnished 

 with lateral fasciculi or tufts, and branchiae; mouth placed 



