234 COMPARATIVE ANATOMY 



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 

 be loaded with bright yellow glistening globules, evidently of 

 a fatty nature, for they blacken with osmic acid. The endoderm 

 cells lining the proximal moiety of the gastrovascular cavity, 

 and, in a starving Hydra, all the endoderm cells, are vacuolated 

 to such an extent that the protoplasm is reduced to a mere 

 envelope forming the outside of the cell and a loose network 

 around the nucleus. The internal free end of the cell, 

 turned towards the gastrovascular cavity, has no cuticle, and 

 the naked protoplasm often sends pseudopodial processes into 

 the cavity. Normally each cell bears a pair of fairly long 

 flagella, whose lashing movements may be observed in a 

 transparent living specimen subjected to pressure. It seems 

 probable that the flagella can be withdrawn and pseudopodia 

 protruded in their place, but nothing is certainly known on 

 this head. At its outer end, abutting on the mesogloea, the 

 nutritive cell widens out suddenly and gives rise to a muscular 

 fibre resembling in all respects that of an ectodermic cell, 

 except that it is shorter, rather finer, and disposed transversely 

 to the long axis of the body. Hence whilst the ectodermic 

 fibres form a longitudinal, the endodermic fibres form a circular 

 muscular coat. The endoderm cells of the tentacles do not 

 appear to have muscular fibres. 



A nutritive cell of H. viridis would be exactly the same as 

 the one described except that the yellow plastids would be 

 replaced by much more numerous chromatophors. 



The endoderm cells are closely packed together, leaving 

 room for only a very few interstitial cells between their basal 

 ends. Being wider at their free ends than at their bases, they 

 are packed in bunches which look fan-shaped in section, as is 

 shown in the diagram fig. 48, A, When the animal is con- 

 tracted the endoderm cells are much elongated and the bunches 

 separated by deep valleys, project far into the gastrovascular 

 cavity. When the animal is extended the cells are correspond- 

 ingly stretched out, the valleys become shallow, and the whole 

 endoderm has the appearance of a rather irregular columnar 

 epithelium. 



The gland-cells are absent from the endoderm of the 



