24 NATURAL HISTORr OF 



derm is a distinct, delicate, structureless membrane 

 called tlie " supporting lamella/' described by Mr. 

 J. T. Parker in the Proc. Roy. Soc, 1880. The muscular 

 layer, with its supporting membrane, is called " the 

 mesoderm." It bas been found that the thread cells 

 are connected with the mesoderm by fibrils. These 

 are considered by some observers as muscular, and by 

 their contraction and extension supplying the neces- 

 sary force for the ejection of the filaments.* Others, 

 however, consider that they are only supporting 

 fibres. 



The cells of the endoderm are arranged in a single 

 layer, and are large and nucleated ; the base is some- 

 what flattened, but the free end is rounded and furnished 

 with cMia, which are in constant motion, and serve to 

 keep in circulation the fluids in the interior of the sac. 

 Dr. A. Weismannf has pointed out that the circulation 

 of the fluids is also aided by rhythmical contractions 

 or pulsations of the body wall. This applies to hy droids 

 generally. The endoderm cells in the hydra are also 

 noticeable from the fact that in one species, H. viridis, 

 they contain a green colouring matter which has been 

 clearly ascertained to be chlorophyll. This fact has 

 been rendered certain by spectrum analysis and other 

 methods.^ As chlorophyll has until recently been 

 considered an exclusively vegetable product, many 

 observers hold that the green colouring matter is 

 caused by minute algse which have penetrated into the 

 substance of the hydra. This view, however, does not 

 accord with the results obtained by the majority of 



* See J. R. M. S., 1882, p. 200. 

 t Zool. Anzeig. iv. (1881), 61—64. 

 X Semper, " Animal Life," p. 83. 



