18 Couthouy's New Species of Mollusca 
Their locomotion is “extremely slow, and is per- 
formed by successive contractions and extensions of the 
body. Their progress is marked by a thick viscid 
substance, which is emitted in great quantities. In 
changing their position, they extend the branchie and 
tentacula in all directions, and if any obstacle is en- 
countered, contract them again very suddenly. If the 
irritating cause continues, the branchie are frequently 
withdrawn, as it were into the body, so as to render 
them nearly invisible, and their reappearance is very 
gradual. 1f the animal is left undisturbed, it glides 
along a short distance and repeats the experiment. This 
happens upon uniform surfaces; but in proceeding from 
one bunch of weed to another, they relinquish their hold 
by degrees, folding the foot together as they do so, till 
they remain attached only by a small portion posteriorly, 
when they reach out in every direction till a new sup- 
port is encounteréd, which they embrace and then sur- 
render the old one. This procedure is exceedingly 
slow, it frequently occupying them ten minutes or more 
to move half as many inches. 
Like the animal of Lymnza and some other genera, 
the Tritonie possess the faculty of moving along the 
surface of the water in an inverted position ; in doing 
which the branchiz appear to be used for the purpose of 
assisting their natation. Upon touching one of them, 
while crawling about in this manner, it was perceived 
that the surface of the water, above its foot, was coated 
with the viscid substance emitted by the animal, by means 
of which it formed a sort of air vessel between its foot and 
the surface, thereby enabling it to suspend itself in this 
singular manner. "Phat this was the mode of effecting 
its purpose, was evident from the creature's sinking in- 
