STRUCTURE OF POLYZOA AND RADIATA. 
123 
arms do not serve, however, like those of polypes, to grasp 
the food; but the vibration of their cilia produces a powerful 
%. JI current which brings both food and oxygen. 
The mouth leads by a large funnel-shaped 
oesophagus or gullet, to a gizzard, b ; in which 
the particles of food that enter it are ground 
down, by the action of its muscular walls and 
of the tooth-like processes that line it. Below 
this gizzard is the true digestive stomach, c, 
around which the rudiment of a liver may 
be traced ; and from this stomach there passes 
upwards an intestinal tube, which terminates 
by a distinct orifice at d, on the outside of the 
circle of arms. The digestive apparatus is evi¬ 
dently formed, therefore, upon a much higher 
plan in these animals than it is in the true 
polypes, which have no true anal orifice. The 
Molluscan character of these animals is further 
shown by the presence of a single nervous 
ganglion, situated between the two orifices, 
as in the Tunicata; this acts upon a complex 
a, oesophagus ; b, giz- apparatus of muscles, by which the animal 
^orifice^f^ntes- can be cither drawn into its cell or projected 
tine* forth from it, with great rapidity. 
116. The fourth subdivision, that of Radiata, includes 
those animals which have the parts of the body arranged in 
a circular manner around a common centre, so as to present a 
radiated or rayed aspect. This arrangement is well seen in 
the common Star-fisk (fig. 65), which has five such rays, all 
having a precisely similar structure, and thus repeating each 
other in every respect. The mouth of this animal is in the 
centre; and it opens into a stomach, which occupies the cen¬ 
tral disk, and sends prolongations into the rays. The nervous 
system is, in like manner, composed of a repetition of similar 
parts. A plan of it is seen in fig. 66; where a shows the 
position of the mouth, which is surrounded by a ring or 
nervous cord, having five ganglia, corresponding to the five 
arms. From each of these ganglia proceeds a branch along 
its arm, terminating in a little organ at its extremity, which 
is believed to be an imperfectly-developed eye. FTo other 
organs of special sense can be detected in any of these ani- 
Fig. 64. 
Bowerbankia. 
