80 POPULAR BRITISH CONCHOLOGY. 



when fully extended, is at least one-third larger than the 

 shell, passing considerably beyond it in the free part, which 

 is thin and transparent, with a smooth, undulating margin. 

 Between this and the shell there is a fringe of filaments, ex- 

 tending completely round to the umboes, but largest and 

 most conspicuous on the back. There is however a single 

 filament much stouter and longer than the rest, which, 

 when the animal is crawling, is waved to and fro, like a 

 tentacle, apparently feeling the way as it goes. The other 

 filaments are comparatively motionless, floating loosely in 

 the water, though capable of being thrown out or withdrawn 

 at pleasure. The mantle is open for about three-fourths of 

 its circumference, being united for a short space before and 

 behind ; in the latter part it forms a short siphon, with a 

 single aperture. The foot is very large, rather thick and 

 tapering in front, with a slight nipple-like termination; be- 

 hind, it extends into a long, pointed heel, with thin margins, 

 which are divided for about half the length of the base, 

 forming, in that part, a kind of disc, the edges of which can 

 be either closed or expanded, as in the foot of the Nucula. 

 As may be imagined from the size of the foot, it has the 

 power of crawling about very freely, and sometimes it also 

 swims inverted on the surface of the water, in the manner 



