CHAPTER IX. 



Details of Ccelenterata. 



I. Hydrozoa. 



A. Hydrida. 



THE Hydras, as a rule, are not coloured in our sense of the term ; 

 that is to say, they are of a general uniform brown colour. 

 But in one species, H. viridis, the endoderm contains granules of a 

 green colour, which is said to be identical with the green colouring 

 matter of leaves (chlorophyll). This does not occur in all the cells, 

 though it is present in most. The green matter occurs in the form 

 of definite spherical corpuscles, and these colour-cells define the 

 inner layer of the integument (the endoderm), and render it distinct.* 

 That portion of the endoderm which forms the boundary of the body- 

 cavity has fewer green corpuscles, but contains irregular brown 

 granules, thus roughly mapping out a structural region. 



We thus see that even in so simple a body as the Hydra the 

 colouring matter is distributed strictly according to morphological 

 tracts. 



B. Tubularida. The Tubularian Hydroids are the subject of an 

 exhaustive and admirably illustrated monograph by Prof. J. Allman, 

 from which the following details are culled. These animals are with 

 few exceptions marine, and consist either of a single polypite or of a 

 number connected together by a common flesh, or coenosarc. Some 

 are quite naked, others have horny tubes, into which, however, the 

 polypites cannot retreat. The polypites consist essentially of a sac 

 surrounded with tentacles ; and one of their most striking characters 



* Allman's Hydroids. Eay. Soc, p. 123. 



