28 HYDRA. 



lamella, and partially imbedded in its substance. 

 Their direction is mainly longitudinal, i.e. 

 parallel to the long axis of the animal, and it is 

 probably on them that the contractility of the 

 animal (p. 18) depends. 



b. The interstitial cells are much smaller, and usually 



remain in small clusters. Within some of them 

 the early stages of development of the nemato- 

 cysts may be seen. 



c. The nematocysts. In specimens killed with acetic 



acid, nematocysts are frequently met with in 

 which the thread is only partially protruded, 

 and in which the relations of the capsule to the 

 thread can be easily determined. 



d. The nerve cells. The small stellate cells supposed 



to be nerve cells may sometimes be seen. They 

 are better seen in specimens treated with methy- 

 lene blue. 



2. The supporting lamella may be seen in teased prepara- 



tions in the form of transparent shreds or sheets of 

 greater or less extent, on the outer surface of which 

 a distinct longitudinal striation is present, caused by 

 the muscular tails of the large ectoderm cells. 



On the inner surface of this lamella trans- 

 verse fibres occur, which appear to be connected 

 with the endoderm cells, and are probably muscular 

 in function. 



3. The endoderm cells are very variable in shape, and 



during life some of them are amoeboid. Each 

 commonly contains one or more non-contractile 

 vacuoles, which may be so large as to reduce the pro- 

 toplasm of the cell to a very thin peripheral lamella. 

 The outer end of each cell contains in Hydra viridis 

 a number of small cells coated with chlorophyll, to 

 which the green colour of the animal is due. In 

 the brown species of Hydra similar bodies are 

 present, but devoid of chlorophyll. 



