24 MOLLUSCA. 
always pushed backwards—a circumstance that 
has misled some naturalists to describe the posterior 
aperture for the true mouth. The Pyrosomata are a 
still more singular family of the same order. Hach 
seeming individual of this genus is, in fact, a 
numerous colony of little mollusca, every one in 
its own cell, distinct, yet inseparably connected 
with its fellows. Collected imto the figure of a 
gelatinous cylinder, open at one extremity and 
closed at the other, and roughened externally by a 
multitude of tubercles disposed sometimes in rings 
and sometimes irregularly, they float in the Aus- 
tralian seas like stars of this lower world, shedding 
around them a halo of light, brilliant indeed, but 
surpassed in beauty by those other colours of the 
creatures which it serves to disclose; colours which 
come and go at pleasure, glorying as it were, in 
their subtle changes, passing rapidly from a lively 
red to aurora, to orange, to green, and to azure 
blue; a magic scene, compelling more than the 
admiration of every beholder.” * 
Bivalve Mollusca in general have much less 
power of shifting their locality than Univalves. 
Many appear to be absolutely stationary, at least. 
during their adult existence. But others, as the 
Cockle, have a most versatile organ, known as the 
foot, capable of being protruded from between the 
valves, which, among its various uses, serves the 
purpose of locomotion. It is in general applied in 
this manner. Being stretched out to its utmost 
extent, its point is made to hook downward into 
the sand or mud, and the body with the shell is 
then dragged down by the muscular contraction of 
the foot. In most cases, this mode of progression 
* Johnston’s Intr. to Conch. p. 124. 
* 
