262 COMPARATIVE ANATOMY 



including the cavities of the tentacles, and consists primarily 

 of two kinds of cells, epithelio-muscular digestive cells and 

 secretory cells devoid of muscular processes. 



The character of the endoderm differs in different regions of 

 the body. In the tentacles it consists wholly of much-vacuolated 

 epithelial cells which are prismatic and columnar when the 

 tentacles are retracted, flattened and elongate-oval in section 

 when the tentacles are extended. Each cell contains a single 

 large nucleus with a conspicuous nucleolus, and, in addition, 

 a number of rounded corpuscles and granules. The round 

 corpuscles in H.fusca are yellow in colour ; they stain tolerably 

 deeply with certain dyes but do not show any trace of internal 

 structure. They are specialised protoplasmic structures, 

 capable of multiplying by division, and are connected in some 

 way unknown to us with the metabolism of the cell. In 

 Hydra viridis the corpuscles are very numerous and bright 

 green in colour. Each consists of a central protoplasmic body 

 containing one or more clear spaces with a minute central 

 corpuscle, which resembles a nucleolus not only in appearance 

 but also in the fact that it stains very brightly with certain dyes. 

 The central body is covered by an envelope of a somewhat 

 different protoplasmic substance containing chlorophyll. The 

 envelope is sometimes continuous, sometimes in the form of 

 two or three cap-like plates. The whole corpuscle recalls the 

 chromatophors found in so many Protozoa, and may be called 

 by the same name. Experiments have shown that Hydra 

 viridis is able to decompose carbonic acid in sunlight, setting 

 free bubbles of oxygen gas. It is not known in what form 

 the carbon is assimilated, for if an animal which has been 

 exposed for several hours to bright sunlight is killed, placed 

 in spirit to dissolve out the chlorophyll, and treated with 

 iodine, no trace of starch can be discovered either in the 

 chromatophors or in the endoderm cells in which they are 

 contained. 



A single nutritive endoderm cell from the body of H. fusca 

 is shown on fig. 55, E. It is an elongate claviform epithelio- 

 muscular cell containing a large nucleus with a nucleolus and 

 a number of granules and corpuscles of different kinds. Some 

 of these are yellow plastids, others are spherical balls made up of 

 a number of highly-refringent angular brown particles, prob- 

 ably products of metabolism. After a meal such a cell will 



