154 



AN INTRODUCTION TO ZOOLOGY 



human being, produce a result similar to the sting of a nettle, only 

 it may be a great deal more severe. 



When once discharged the nematocyst thread cannot be intro- 

 verted again, and so is useless and shed. They must, therefore, be 

 replaced constantly, and so we can find in the interstitial cells, 

 especially in the distal parts of the body, many that have capsules 

 in different stages of formation within them. They are used up most 

 rapidly on the tentacles where, however, there are no interstitial 

 cells, and it is not quite clear how the replacement is effected. It is 

 almost certainly brought about by the migration of the fairly young 

 cnidoblasts, although this has not been demonstrated. Two 

 varieties of nematocyst are met with in Hydra ; one, the larger, is 

 of an oval form similar to that described above, and the other is 

 smaller, a longer oval, and the thread, when it is discharged, is 

 thicker, shorter and without barbs at the base. 



The nematocyst is a structure confined to the Ccelenterata, but 

 widely, if not universally, distributed in that phylum, and retaining 



throughout the same 

 fundamental structure, 

 although the size, pro- 

 portion of its parts, and 

 the presence or absence of 

 barbs varies from species 

 to species. It forms, as 

 can readily be seen, a very 

 efficient weapon. 



2. The second type of 

 cell produced by the in- 

 terstitial cells is the nerve 

 element. All the cells of 

 Hydra appear to be very 

 sensitive, but the nerve 

 cells are those in which 

 this property is most 



highly developed. They are small, bipolar or multipolar primitive 

 ganglion cells, and all their processes may branch freely, but do not 

 appear to be structurally differentiated into axons and dendrons. 

 Around the mouth they are rather more numerous than elsewhere, 

 but even here they do not form ganglia or aggregations, being merely 

 scattered about in the mesoglea. In this way they form a loose 

 network of a nervous nature, serving to keep all parts of the animal 

 in communication with one another, and so co-ordinating the 

 activities of the animal as a whole. 



3. A few cells on the hypostome and the basal disc appear to b 



FIG. 51. -Primitive nerve cells of Hydra, 

 adapted from Schneider. 



