MOLLTJSCA. 165 



their dorsal surface scattered plumose branchiae. Each of 

 the tentacula is furnished with a large funnel-shaped sheath. 

 The foot is very narrow, with a mesial groove, used in 

 climbing up the stalks of sea- weeds. The mouth is placed 

 at the base of the tentacula, and surrounded with a semi- 

 circular lip. The tongue is in the form of a tubercle, with 

 reflected points. The gullet is plaited longitudinally. The 

 stomach is short and cylindrical, with a ring of hard, longi- 

 tudinal scales. The liver consists of six unequal globules, 

 and the bile is poured into the cardiac extremity of the gul- 

 let. The Scyllea pelagica has been long known to naturalists, 

 and appears to be very common in the equatorial seas, ad-^ 

 hering to the stems of the Fucus natans. 



GENUS THETHYS. Branchiae forming a row on each side 

 of the back, consisting of fringed processes, alternately larg- 

 er and smaller. The body is ovate, with the cloak and foot 

 continuous. The neck is distinct from the foot, and is nar- 

 row. Above, the neck is continuous with the cloak, from 

 which arises a large semicircular expansion, used probably 

 as a fin. The margin of this expansion is fringed with nu- 

 merous filaments, and on the upper surface, within the 

 border, is a row of conical tubercles. The true tentacula 

 are placed towards the base of this fin near the neck. Each 

 of them consists of a small fleshy cone, striated across, with 

 a semicircular sheath behind. The branchiae consist of a 

 tapering, fleshy stalk, spirally twisted towards the summit 

 with a series of filaments on one side. They are fourteen in 

 number on each side,alternately and oppositely small and large. 

 The anus opens in front of the third branchia on the right 

 side. The orifice of generation is exhibited under the first 

 branchia of the same side. In front of each of the larger 

 branchiae, is a small cavity with a .small filament in the cen- 



