24 HYDEA. 



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. 20) 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 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 mesogloea may be seen in teased preparations 

 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 mesoglcea trans- 

 verse fibres occur which appear to be connected 

 with the endoderm cells, and are probably muscular 

 in function. 



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

 during life are amoeboid. Each commonly con- 

 tains 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 each cell, 

 next to the mesogloea, 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. 



