35 



owinjv to tlio innutnovablo quautity of monads, thouj^h in- 

 cliviaually thoy aro perfectly invisiljlo to tlio luikeil oyo. Ou 

 these inouarls the pi umat-lla fuel voraciously, and as they aro 

 reduced by the action of tlio stomach iu tho process of di^restion, 

 the stomach becomes filled with a brig-ht grcon-colourod matter. 

 By roferniig to my diagram you will see that tho mouth of tho 

 plumntoUa is surroumled by a number of teiitacula, arising from 

 a sort of stage or disk, termed tho lophophore. These teutacles 

 are covered with vibratile cilia, which, M-hen in motion, have the 

 appearance of passing up on one side of each tentacle and down 

 the oi:)posite, the rapid motion of which causes a current of water 

 to set in, in tlio direction of the mouth, bearing- with it the 

 food requisite for the support of the animal. The whole course 

 of the alimontary matter tlius obtained, from the moment of its 

 prehension to its iinal ejection, may be easily witnessed in many 

 ot tho fresh-wa^er polyzoa. If a polypide of plumatella repens 

 be watched, while in an exerted state, different kinds of infusoria 

 and other minute organic bodies may Ijo observed to be whirled 

 along m the vortices caused by the acti(m of the tentacular cilia, 

 and conveyed to the mouth, where many of them are at once 

 seized and swallowed and others rejected. The food having once 

 entered the oesophagus experiences in this tube no delay, but is 

 rapidly conveyed downwards by a kind of peristaltic action, and 

 delivered to the stomach. In the stomach the food is destined 

 to experience considerable delays ; it is here rapidly moved up 

 and down by a strong peristaltic action, which first takes place 

 from above downwards, and then inverting itself, propels the 

 contents m an opposite direction. Every now and then the 

 fundus of the stomach seems to perform some function distinct 

 from that of the rest of the organ, in that it seizes a portion of 

 tlie alimentary mass and retains it for a moment, by an hour- 

 glass restriction, separate from the remainder, and then power- 

 fully contracting on it, forces it back among tho other contents 

 ot the stomach. All this time the food is becoming imbued with 

 the peculiar secretion of the gastric walls, and soon assumes a 

 rich green colour. After having undergone for some time tho 

 action of_ the stomach, tho alimentary matter is delivered by 

 degress into tho intestine, where it accumulates in the wido 

 pyloric extremity of this tube. After continuim? here for a 

 while in a state of rest, ajid probably violding to the absorbent 

 tissues its remaining nutritious elements, portions, in the form 

 of oval-slmpod pellets, become separated at intervals from the 

 mass, and are slowly ])ropelled along tho tube towards tho vent, 

 where, having arrived, they aro suddenly ejected into the sur- 

 rounding water and rapidly M-hirled away by the tentacular 

 currents. In all the fre.sh-water polyzoa, bodies of a very 

 peculiar nature occur at seasons lying loose in the perigastric 



