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TEXT-BOOK OF ZOOLOGY 



their bases. Now, exactly the same thing happens when one of the 

 small crustaceans above mentioned touches the tentacle. Forthwith 

 it is hit by a larger number of these missiles, which, being hollow 

 and conveying a poison from the capsule into the body of the victim, 

 apparently kill or paralyze the latter, which is thus bound helpless 

 to the arm of its assailant. The human skin is too thick to feel 

 the effect of these weapons, though many species allied to the fresh- 

 water polyps can inflict in some cases very 

 painful stings by means of exactly similar 

 organs. In the case of some species the 

 sensation is like being burnt with a red-hot 

 iron, in others like putting one's hand 

 among stinging-nettles. From the stinging 

 action of these organs they have been called 

 thread or nettle cells (cnidse, or nematocysts), 

 and the animals provided with these organs 

 — Cnidaria or Neviatophora (Greek cnide, a 

 nettle ; nema, a thread ; and phoreo, I bear 

 — i.e., nettle- or thread-bearing animals). 



(c) The opening through which we saw 

 the crustacean disappearing into the interior 

 of the polyp is the mouth, which, however, 

 in the absence of a second body aperture, 

 also serves as anus. 



(d) A longitudinal section through the 

 polyp (see illustration) clearly shows that 

 the mouth leads into a spacious cavity, 

 which is continued also into the tentacles. 

 This space corresponds to the body cavity 

 of higher animals in which the viscera are 

 lodged ; secondly, it carries out digestive 

 functions performed in the latter by the 

 intestinal canal, and finally conveys the 

 digested nutriment to all parts of the body, 



which function in the higher animals is carried out by the bloodvessels. 

 Thus, the structure of a polyp is a very simple one, and since all its allies 

 also possess such a single body cavity, they are described as Ccelenterata, 

 i.e., hollow animals (Greek ccelos, hollow, and enteron, the bowel). The 

 walls lining this body cavity are composed of two layers (ectoderm and 

 endoderm), composed of those minute bodies which have been already 

 referred to as cells (Part II., p. 253). Interposed between these two 

 layers is a third very thin layer not composed of cells, and somewhat 



Longitudinal Section through 

 a Fresh-Water Polyp (Dia- 

 grammatic). 



The animal (in order to facilitate 

 comparison with a jelly-fish) is 

 shown attached to a plant-stalk, 

 with its mouth pointing down- 

 wards. A., Two tenacles ; H., 

 body cavity; M.,, mouth; a.S., 

 external layer of integument 

 (ectoderm) ; i.S., internal layer 

 (endoderm) ; St., supporting layer 

 (mesoderm) ; K., bud. 



