452 



INTERNAL STRUCTURE OF ECHINIDA. 



relative position of these orifices undergoes important variations 

 in the different species. The intestinal tube usually makes about 



FIG. 605. ANATOMY OF ECHINUS : a, mouth, surrounded by the teeth and jaws, c, c ; 

 b, oesophagus ; s, stomach, or first portion of the intestine ; d, intestinal tube ; e, ovary ; 

 f, /, ambulacra! vesicles ; g, g, shell. 



two turns within the shell ; and it is attached to its walls by a 

 fold of the membrane that lines the general cavity. This mem- 

 branous fold contains distinct blood-vessels, by which the nutri- 

 tive matter seems to be absorbed from the digestive cavity ; 

 and these blood-vessels form part of a complex system, by which 

 the absorbed fluid is distributed over the whole body. We have 

 in these animals, too, a distinct provision for the aeration of their 

 fluid, by its exposure to a constantly-renewed layer of water. 

 The whole cavity of the shell, except the portion occupied by 

 the intestinal tube and its appendages, seems to be one great 

 respiratory chamber. Water is admitted to it from without ; 

 and its lining membrane, on which the blood-vessels are minutely 

 distributed, is covered with cilia, by the vibrations of which 

 the fluid covering its surface is continually renewed, and 

 the blood thus obtains a constant supply of oxygen from the 



