PREPARED SPECIMENS 23 



tails lie on the outer surface of the supporting 

 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 supporting 

 lamella (p. 19) 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 nematocysts 

 may be seen. 



c. The nematocysts. In specimens killed with Miiller's 



fluid, 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 readily determined. 



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 the supporting lamella 

 transverse fibres occur which are described by some 

 writers as connected with the endoderm cells, and 

 are regarded as muscular in character. 



3. The endoderm cells are very variable in shape. Each 



commonly contains one or more vacuoles which may 

 be so large as to reduce the protoplasm of the cell to 

 a very thin peripheral lamella. The outer end of 

 the cell, next the supporting lamella, contains in 

 Hydra viridis a number of small spherules 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. 

 The endoderm cells may also contain, in addition 

 to particles of food, little clusters of brown or black 

 granules, probably formed by breaking down of the 

 chlorophyll grains. The flagella are seldom pre- 

 served in teased specimens. 



