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POPULAR SCIENCE REVIEW. 
proximate the apices of the five teeth with ease, force, and 
delicacy. 
The muscles by which these movements are effected, and 
which, like those of the lobster, are formed of striated fibres, 
are — 1. Those which connect the adjacent sides of the alveoli, 
which are finely striated to give insertion to these muscles. 
2. Those which pass from the auriculae to the apices of the 
alveoli — i.e. from those arches before mentioned within the 
cavity of the shell at the base of the ambulacra. The two 
muscles of each pair of this category pass from one auricula to 
two alveoli, each alveolus receiving one such muscle from the 
auricula on each side of it. 3. Those which pass from the bases 
of the alveoli to the interspaces of the auriculae. 4. Those 
which connect together the radii; and, 5. Those which pass 
downwards from the apices of the radii to the inside of the shell 
in the interspaces of the auriculae — one muscle passing from one 
arm of the bifurcation, to the shell on the same side of the 
adjacent auricula ; the other muscle passing from the other arm 
of the bifurcation, to the shell on the other side of the adjacent 
auricula. 
The muscles of the categories 3 and 5 approximate the teeth ; 
those of the category No. 2 separate them. 
In the angles opposite the notches of the oral margin are 
placed five branched gills, or plumose external processes of the 
perisoma, attached to the buccal membrane. 
Internally the mouth leads to an alimentary canal of a 
very simple character, without any differentiation into stomach 
and intestine ; the oesophageal part, however, is narrower than 
the rest. This simple canal passes twice round the interior of 
the shell, to the inner wall of which it is suspended by mesen- 
teric filaments. It terminates in the opening at the apical 
pole. There is no liver or any other similar organ to aid the 
process of alimentation. 
The cavity of the body external to the alimentary canal 
contains the true blood, mixed with more or less water. This 
is kept in motion by means of vibratile cilia, which coat the 
inner wall of the shell and the alimentary canal. 
There is no true heart, and there are no true blood-vessels ; 
but there are many vessels of other kinds to be described 
directly. 
The only generally recognised organs for respiration are the 
fine plumose processes round the mouth before mentioned ; the 
influence of these, however, must be exceedingly slight. 
The vessels containing fluid are of two kinds : one of them 
is called the ambulacral system , the other is termed the 
pseudhcvmal system , and has been mistaken for a system of 
true blood-vessels. 
