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 

 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 

 rorifice s ofnfe h s i can be either drawn into its cell or projected 

 tiue. forth from it, with great rapidity. 



116. The fourth subdivision, that of EADIATA, 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-fisti (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. No other 

 organs of special sense can be detected in any of these ani- 



Fig. 64. 



BOWERBANKIA. 



