54 CONCHOLOGIST’S COMPANION. 
his station, the shell is gradually opened by the help 
of this protuberance, which, assuming a new form, 
pushes forward and makes a furrow in the sand, into 
which the shell is drawn in a vertical position. From 
this position he almost immediately changes into his 
former horizontal one; the tentacula shovelling back 
the sand and lengthening the furrow, while the 
animal journeys on his way, with a motion resembling 
a continual topsy-turvy. These tracks, most pro- 
bably formed by the Muscle in quest of food, may 
be readily discerned in shallow clear streams, and 
resemble small furrows upon the sandy bottom; they 
are seldom straight, but deviate into traverses and 
triangles, like the course of a vessel when contending 
with adverse winds. 
Muscles found in the salt springs of. Nubia, 
journey during the rainy season to a considerable 
distance from their usual places of abode. They 
even migrate occasionally so far, that when the 
rains abate, they have neither strength nor sufficient 
moisture left to enable them to return to their com- 
panions. 
Such as inhabit the sea-shore are also furnished 
with the means of progressive motion; they can 
open and shut their shells at pleasure—remove to 
a considerable distance—fasten themselves to the 
rocks with threads similar to those of the silk-worm, 
