

ORGANS OF DIGESTION. 313 



are no such papilla) on those which fix on the under surface ; 

 the vents open on the outer margins of the ramified forms 

 (Fig. 2 b. b.), and they open into the interior of the tubular 

 species (Fig. 3. b.), so as to be most free from the absorbent 

 pores which would readily be obstructed by the mucous 

 flocculi and particles of foreign matter propelled from the 

 large orifices. The absorbent canals of poriphera are like 

 the ramified roots of a plant turned inwards, and from the 

 simplicity and similarity of their structure in every part they 

 are susceptible of infinite division without destroying their 

 vitality, and distinct individuals, by coming into contact in 

 the progress of their growth, easily coalesce to form one 

 mass. 



III. Polypiphera. In the polypipherous animals or zoophy- 

 tes the digestive organs are more distinct from the common 

 cellular tissue of the body, and present a more complicated 

 form than in the porifera, as the margins of the pores are 

 here lengthened out to form little stomachs or polypi, or- 

 ganised to select, and seize, and digest living animalcules ; 

 parts of the lips of these polypi are also still further extend- 

 ed to form sensitive prehensile tentacula, and the sides of 

 these tentacula develope numerous minute filiform cilia, by 

 the rapid vibration of which currents are produced in the 

 water to attract prey. In the hydra or fresh-water polype, 

 the whole digestive apparatus consists of a simple sac, exca- 

 vated in the cellular substance of the body, and destitute of 

 all cceca or glandular appendices, and even of a distinct 

 anus. The parietes of this simple polype appear to possess 

 the same properties in every part, as they continue to seize 

 and digest food when the animal is turned inside out, and 

 each part of the animal, when cut to pieces, is found to 

 develope itself to a perfect polype. What was formerly the 

 internal digestive surface is found also to become the genera- 

 tive, and to produce gemmules and young polypes when the 

 animal is turned inside out. They feed chiefly on larvae and 

 annelides which they search for and seize by the long tenta- 

 cula developed from the sides of their mouth, and they often 

 swallow animals many times larger than their own body, by 

 stretching their thin elastic parietes over their prey. The 

 digested part of the food passes through the common cellular 

 tissue of their body, and through their tubular tentacula,. 



