1 6 Elementary Zoology. 



narrowed towards the base, where they end in long contractile 

 tails which are in effect muscle fibres (Fig. 7). Between these 

 larger cells are heaps of small interstitial cells, some of which 

 are ganglionic in nature and communicate with fine nerve 

 fibrils. When a living hydra is examined intact, fine bunches of 

 bristle-like processes are seen to protrude on the outside ; these 

 are the indications of the thread cells, or, better, cnidoblasts, as 

 they are not cells, which are particularly abundant upon the 

 tentacles. The cnidoblasts (Fig. 8) are formed by the metamor- 

 phosis of the interstitial cells, but when fully formed force their 

 way to the exterior. The fully developed but not used cnidoblast 

 has the characters shown in the diagram. It is enclosed in an 

 interstitial cell which has a delicate process protruding on to the 

 exterior ; this is the cnidocil or palpocil. The function of this 

 trigger-like projection is not, as might be inferred from the 

 vernacular name just used, which is sometimes applied to it, the 

 grossly mechanical one ; it is probably of a nervous nature, like 

 the fine end of &, visual or auditory cell, and communicates an 

 impression to the cell, which then contracts and expels the 

 " thread " from the cnidoblast. The latter is a tough sac formed 

 of a spherical or pear-shaped base, with a fine hollow process ; 

 it has been aptly compared to a glove with one finger, the hand 

 of the glove being, of course, nowhere open. 



The "finger," however, is turned inside out within the 

 " hand," and the whole is filled with fluid. Pressure forces out 

 the thread, which is then seen to be often armed with spines at 

 its base. The " sting " of the jelly-fish is due to similar thread 

 cells ; but whether there is an actual poisonous liquid, which 

 causes the symptoms, or whether it is merely the irritation, like 

 that caused upon sensitive skins by the hairs of certain cater- 

 pillars, does not appear to be certain. From the interstitial 

 cells are also formed the generative tissues, which are rounded 

 swellings nearer the tentacles in the case of the testes, and nearer 

 the base in the case of the ovaries. The testes contain tadpole- 

 like spermatozoa, formed by the divisions of the testicular cells, 

 while the ovary only contains one egg at a time, which is large 

 and full of yolk, and exhibits before it is quite ripe amoeboid 

 movements. The ovum nourishes itself upon the small cells of 



