172 PECTINIBRANCHIATA.—CYPRAHEADA. 
arranged lengthwise. But probably few are aware 
how very elegant a creature it is when tenanted by 
its living inhabitant, and crawling at ease in clear 
water. The foot, on which it glides with a slow 
but smooth motion over the surface of the rock on 
which it habitually dwells, or, if you please, on 
the bottom of the saucer of sea-water in which you 
are examining it, is a broad expansion spreading 
out to twice the superficies of the base of the shell. 
Above this is the fleshy mantle, which is so turned 
up as closely to invest the shell, conforming to its 
shape, and even fitting into the grooves between 
the ridges. This mantle can be protruded at the 
will of the animal, so far that the two sides meet 
along the top of the shell, and completely cover it ; 
or can be completely retracted within the wrinkled 
lips beneath ; and it is capable of all gradations of 
extension between these limits. From the front 
of the shell protrudes the head, armed with two 
straight and lengthened tentacles, answering in 
function and appearance to the upper part of horns 
in a snail; except that the little black points which 
constitute the visual organs are not in this case 
placed at the tips, but on a little prominence on 
the outside of the base of each tentacle. Above 
and between these, which diverge at a considerable 
angle, projects the proboscis, a rather thick, fleshy 
tube, formed by a flat lamina, with its edges bent 
round so as to meet along the under side. The 
interior of this proboscis is lined with delicate 
cilia, by whose constant vibrations a current of 
water 1s drawn into the tube, and poured over the 
surface of the gills for the purpose of respiration. 
This current may be readily perceived by any one 
who will take the trouble to watch with a pocket- 
