260 COMPARATIVE ANATOMY 



distal moiety of the body wall, where the indifferent interstitial 

 cells are most abundant. The exploded nematocysts of the 

 tentacles must be renewed, and since they are not formed in 

 situ, they can only be replaced by the new capsules formed in 

 the distal region of the body wall. How this replacement is 

 effected is not known. It can only be by migration of the 

 cnidoblasts, but the fact of migration has not been satisfactorily 

 established, though as it has been shown that the outer moieties 

 of the epithelio-muscular cells are confluent and form a practic- 

 ally continous sheet of protoplasm, there appears to be no 

 obstacle to migration. 



In the late spring and summer months, when Hydra repro- 

 duces itself sexually, the indifferent interstitial cells at certain 

 defined spots multiply rapidly by division and form masses of 

 germ cells bulging out the ectoderm. Their further develop- 

 ment will be described later on. 



Observation of a living Hydra has shown that it is highly 

 irritable. Irritation of the tentacles will generally cause the 

 whole animal to contract. This would seem to imply the 

 existence of a mechanism whereby impulses generated at one 

 jpart of the body may be conducted to all other parts and cause 

 the muscle-fibres of the epithelio-muscular cells to contract in 

 unison. In short, it would seem to imply the existence of a 

 nervous system. It is impossible to find a trace of a nervous 

 system in sections. Nor has the writer been able, after numerous 

 attempts with the aid of the most approved methods of isola- 

 tion, to demonstrate the existence of structures which could 

 unhesitatingly be called nerve-fibres or nerve ganglion-cells. 

 It is extremely doubtful whether any differentiated nervous 

 elements occur in Hydra, but a nervous system has been 

 described in the form of a sub-epithelial network of branching 

 cells whose processes form a layer of varying thickness lying 

 close on the muscle-fibres in all parts of the body. Some of 

 the branches are said to run upwards between the epithelio- 

 muscular cells and end in or upon them ; others to make a 

 similar connection with the cnidoblasts. These nerve cells are 

 said to be found between the lower ends of the endoderm as 

 well as the ectoderm cells, and to be so abundant on the hypo- 

 stome as to form an ill-defined nerve ring near the bases of 

 the tentacles. 



The ectoderm of the tentacles consists almost exclusively 



