DIGESTIVE SYSTEM. 45 



poda, there is no alimentary canal. The substances received 

 as food, by the extensions of the sarcodous mass, may be re- 

 tained at any portion of the organism during the digestion of 

 its nutritive elements; the excreta to be subsequently thrown 

 off at a convenient point. With certain ciliated Infusoria, the 

 digestive system is wanting, as in Trichomonas. In Paramedum 

 the food enters an oral opening, and is passed along a short 

 gullet to the interior of the body, where, in the form of globu- 

 lar masses, it is digested. The ejecta escape by the mouth. 



RADIATA. Ccelenterata. Digestion takes place within a 

 central cavity. 



Polypi. Cavity simple, as in Hydra, or divided into cham- 

 bers by longitudinal plications of the outer wall, as in Actinia. 

 The wall of the cavity may be separated from integument by 

 an intervening space (perigastric), which is commonly trav- 

 ersed from within outward by partitions defining a number 

 of cells. These communicate with stomach by apertures 

 closed during digestion, but which subsequently open to 

 allow the escape of the chyle into the perigastric space. In 

 the coralline polyps this space being common to many indi- 

 viduals, chyle may be conveyed to a distance from the cavity 

 elaborating it. 



Acalephse. Cavity large, as in Beroe, or small, as in Pleu- 

 robranchia. It is commonly furnished with thickened parti- 

 tions, which transform the chambers into tube-like canals. 

 The oral openings are often prolonged into pendant tenta- 

 cles, which in Physalia (Portuguese man-of-war) are long and 

 numerous. 



Echmoderrnata. The digestive apparatus is separated as 

 a tube or sac, having a distinct oral and commonly an anal 

 opening. When tubular, it is simple, enclosed in a peritoneal 

 covering, and longer than body, as in Toxopneustes (sea- 

 urchin), or as long as body, as in Synapta. When sac-like, 

 it is simple, with numerous short cseca not entering rays, 

 as in Opkiura, or with cseca extending along entire length 

 of rays, as in Asteracanthion (star-fish). The stomach is sepa- 

 rated into two chambers by a circular projecting fold. The 

 first of these is the true stomach, and the second sends off 



