NUTRITION IN THE ACTINIA. 



15 



particles brought within reach of the mouth by means of 

 currents excited by the motions of the cilia. 



Such are the simple forms assumed by the organs of as- 

 similation among the lowest orders of the animal creation; 

 namely, digesting cavities, whence proceed various canals, 

 which form a system for the transmission of the prepared 

 nourishment to different parts; but all these cavities and ca- 

 nals being simply hollowed out of the solid substance of the 

 body. As we ascend a step higher in the scale, we find 

 that the stomach and intestinal tube, together with tlieir 

 appendages, are distinct organs, formed by membranes and 

 coats proper to each, and that they are themselves contained 

 in an outer cavity, which surrounds them, and which re- 

 ceives and collects the nutritious juices after their elabora- 

 tion in these organs. The t^ctinia, or Sea Jlnemone, for 

 example, resembles a polypus in its general form, having a 

 mouth, which is surrounded with tentacula, and which leads 

 into a capacious stomach, or sac, open below, and occupying 



the greater part of the bulk of 

 the animal; but while, in the 

 polypus, the sides of the sto- 

 mach constitute also those of 

 the body, the whole being one 

 simple sac; in the actinia, spaces 

 intervene between the coats of 

 the stomach, and the skin of the 

 animal. As the stomach is not 

 a closed sac, but is open below, 

 these cavities are, in fact, continuous with that of the sto- 

 mach: they are divided by numerous membranous parti- 

 tions passing vertically between the skin, and the membrane 

 of the stomach, and giving support to that organ. Fig. 257, 

 representing a vertical section of the Jictinia coriaccd, dis- 

 plays this internal structure, b is tlie base, or disk, by 

 which the animal adheres to rocks: i is the section of the 

 coriaceous integument, showing its thickness: m is the cen- 

 tral aperture of the upper surface, which performs the oflice 



