2 3 o COMPARATIVE ANATOMY 



furrows and wrinkles, the cuticle dips down into the valleys 

 between the wrinkles. To sum up, the ectoderm of Hydra is 

 mainly composed of columnar epithelial cells, differing from an 

 ordinary epithelium chiefly in the fact that each cell has formed 

 at its base one or more long contractile muscular fibres. Each 

 cell, then, partaking of the characters of an epithelial and an 

 unstriped muscular cell, is called an epithelio-muscular cell. 

 It should be borne in mind that the muscle fibres are relatively 

 very long, and that therefore they extend upwards and down- 

 wards among the similar muscle-fibres of adjacent cells, 

 forming a regular longitudinal layer of muscle-fibres closely 

 applied to the mesoglcea. 



The other elements of the ectoderm of the body wall are the 

 sub-epithelial or interstitial cells and the nematocyst-forming 

 cells or cnidoblasts. As the epithelio-muscular cells taper 

 towards their bases spaces are left between them which in the 

 distal moiety of the body wall are occupied by numerous round 

 or cubical cells, each having a nucleus with a nucleolus. Their 

 nuclei are considerably smaller than those of the epithelio- 

 muscular cells. These rounded cells may be called indifferent, 

 because they are as yet undifferentiated and are destined to 

 give rise to other elements, cnidoblasts and sexual cells. 

 Frequently they may be seen in active division, and their 

 products may be observed to develop into cnidoblasts or into 

 ova or spermatozoa. The majority go to form cnidoblasts. 

 A nematocyst, whether of the large oval, cylindrical or small 

 oval kind, is always the product of a single cell, and is formed 

 inside that cell. In the upper part of the body wall one can 

 see nematocysts in all stages of growth. First a clear space 

 appears in an interstitial cell ; this space enlarges, it acquires 

 a definite wall, and its contents stain deeply. Presently it 

 elongates, and one end is produced to form the thread, which 

 at its first appearance is everted and coiled round the outside 

 of the sac. After a time the thread is introverted it is not 

 quite clear how and the nematocyst assumes its final form. 

 When nearly ripe a nematocyst, still contained in its mother- 

 cell or cnidoblast, migrates into the inside of an epithelio- 

 muscular cell and approaches the surface. The external end 

 of the cnidoblast is produced to form a cnidocil which per- 

 forates the cuticle, and the gun is, so to speak, loaded and 

 cocked, ready to explode on the pressure of the trigger the 



