230 THE VOYAGE OF H.M.S. CHALLENGER. 



L M. Longitudinal muscular fibres seen in section as a series of dark 

 points. 



G. Gastric cells. 

 The narrow dark zone between the longitudinal muscles and the membranous 

 layer indicates possibly a circular muscular layer. 

 Fig. 8. Small portion of a vertical section of the ccenosarc, much enlarged, showing 

 the histological structure of the vascular network. The vessels are seen 

 cut open in almost their entire course. The walls of the deeper vessels are 

 very thin, and these vessels are filled with transparent spherical globules. 

 More superficially the walls of the vessels become thickened, and the cells 

 composing their ectodermal layer are seen in several places to be in process 

 of development into thread-cells. At the actual surface the cells of the 

 ectoderm assume an elongate prismatic form. The vessels of the more 

 superficial parts of the network are filled with the pigmented cells, 

 mingled with transparent globules. 



E. Superficial layer of the ectoderm. 



M. Membranous layer of the ccenosarcal canal. 



C, C. Pigmented cells lying in the cavities of the canal. 



B. Transparent globules filling the deeper ramifications. 



T, T. Developing ovoid thread-cells. 



T'. Developing thread-cells of the form peculiar to Hydrozoa. 



S. Band of gelatinous tissue passing between the walls of two neigh- 

 bouring vessels. 



A, A. Spaces occupied in the recent condition by calcareous matter. 



A'. Such a space in the superficial ectodermal layer. 



0. Opening in a vessel cut at right angles to its course. 

 Fig. 9. Pigmented cells, of which the endoderm of the ccenosarcal vessels is mainly 

 composed, and which are abundant also within the body-cavities of the 

 zooids. a, d, d Examples of the cells, showing various forms and arrange- 

 ments of the pigmented granules and vesicles which compose their contents. 

 c Cell showing a division of its contents into two. b Cell showing a further 

 division of its contents into four. 



