310 ORGANS OF DIGESTION. 



anenterous monad with a single cavity presents the simplest 

 form of the digestive apparatus known among animals. In 

 some of the larger paramfecia the food appears to move freely 

 in round masses through the general internal cavity of the 

 body, and these are sometimes accumulated at one end of 

 the animal and sometimes at the other. The straight ali- 

 mentary canal with its numerous lateral appendices in the 

 orthoccela, approaches most nearly to that of many hel- 

 minthoid articulata, as the halithea, the leech, and many 

 inferior forms of annelida, rotifera, and entozoa. The open- 

 ing of these coeca on the surface of the body, and changing 

 the direction of the food's motion, would produce the form 

 of alimentary organs presented by the poripherous ani- 

 mals. 



II. Poriphera. The alimentary apparatus of poripherous 

 animals, by the peculiarity of its form and the simplicity of 

 its structure, approaches the nearest to that of plants ; the 

 cellular tissue of their body is permeated in all directions by 

 ramifying and anastomosing canals, which begin by minute 

 superficial pores closely distributed over every part, and 

 terminate in larger orifices variously placed according to the 

 exterior form of the entire animal. In the minute superficial 

 absorbent pores we can generally perceive a fine transverse ge- 

 latinous net-work (Fig. 2.N.) an dprojectingspicula, to protect 

 these entrances from the larger animalcules and from noxious 

 particles floating in the water. The internal canals, like the 

 veinous system, leading from capillaries to trunks, are bound- 

 ed internally by a more condensed or more highly organised 

 portion of the general cellular substance of the body, and 

 are incessantly traversed by streams of water, passing in- 

 wards through the minute pores, and discharged through the 

 larger orifices or vents ; but no polypi have been observed in 

 any of those parts, nor even cilia, although from analogy we 

 may suppose them necessary as the active agents of the 

 currents. In this simple organization there appears to be 

 only an increased extent given to the general cutaneous 

 absorbent surface; there are yet no distinct cceca or 

 stomachs for receiving and retaining the aliment that has 

 been conveyed into the body along with the currents 

 of water. These animals in their earliest free and 

 moving condition, while they are in the state of gemmules. 



