128 STRUCTURE OP ECHINODERMATA AND ACALEPH^. 



to allow of the separation of its two halves. The mouth, &, 

 situated at one of the poles of the shell, is surrounded by a 

 very curious apparatus of jaws and teeth (fig. 69), which 

 forms what is termed the "lantern;" from the mouth com- 

 mences the long narrow oesophagus, m, that leads to the 

 stomach, n, which is merely a dilated portion of the fl.1imp.n- 

 tary tube ; the continuation of this, o } <?, r, forms the intestinal 

 canal, which winds once round the shell, and then doubles 

 back and winds in the opposite direction, terminating at the 

 anal orifice, s, which is situated at the opposite pole. The 

 intestine is held in its place by a double fold of "the mem- 

 brane lining the shell, resembling the mesentery of higher 

 animals ; the blood is distributed over this membrane, to be 

 exposed to the aerating influence of the water admitted into 

 the cavity of the shell ; and the water is kept in movement 

 by the cilia with which the membrane is clothed. Round the 

 anus, s, are seen the five branching ovaries, each of which 

 discharges its contents by a distinct orifice. The circulating 

 apparatus is imperfect, the blood not being impelled by a 

 distinct heart ; still, however, it moves in great part of its 

 course through proper vessels, and not through mere chan- 

 nels in the tissue. In the Star-fish, however, the body is 

 very much flattened ; and the stomach, instead of having a 

 separate intestinal tube with a distinct orifice, is a mere bag 

 with a single aperture, which serves both to take in food and 

 to cast forth the indigestible remains. This character will be 

 found to prevail among all the inferior Radiata. 



120. The radiated structure is also well seen in the greater 

 number of animals forming the group of ACALEPH.E, or Sec,* 

 Nettles. Their bodies are entirely soft and jelly-like ; contain- 

 ing so small a quantity of solid matter, that, when upon being 

 taken out of the water their fluid drains away, there is 

 scarcely anything left; hence they are commonly termed 

 Jelly-Fish. They derive their other name of Sea-Nettles from 

 the stinging power which most of them possess. They are 

 formed to float freely in the water; but they do not in 

 general possess any means of actively propelling themselves 

 through it. The radiated arrangement is very regularly pre 

 served in some of this group, whilst it is less evident in 

 others. The accompanying figure (fig. 70) represents one of 

 the Medusa tribe, as seen floating in water. The umbrella- 



