STRUCTURE OF A SPECIES OF MILLEPORA. 
135 
Fig. 16. Small portion of a vertical section of the hydrophyton, 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 sur- 
face the cells of the ectoderm assume an elongate prismatic form. The 
vessels of the more superficial parts of the network are filled with the pig- 
mented cells, mingled with transparent globules. 
E. Superficial layer of the ectoderm. 
M. Membranous layer of the hydrorhizal vessels. 
C, C. Pigmented cells lying in the cavities of the vessels. 
B. Transparent globules filling the deeper vessels. 
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. 
O. Opening in a vessel cut at right angles to its course. 
Fig. 17. Pigmented cells, of which the endoderm of the hydrorhizal 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 arrangements 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. 
