x.\ : x THE VOYAGE OF H.M.S. CHALLENGER. 



Medusae, lines the entire extent of the hollow space of the gastrovascular system, consists 

 everywhere of a simple layer of flagellate cells, and passes into the ectodermal epithelium 

 only at the oral margin. Corresponding to the shape of the umbrella enclosing the gastral 

 space, we can also distinguish in this endoderm two different principal parts contiguous 

 at the umbrella margin, the dorsal epithelium, and the ventral epithelium of the gastro- 

 vascular system. Both show striking and constant differences. The dorsal endoderm (or 

 the " gastral epithelium of the notumbrella," often also simply termed the " umbral or 

 upper endoderm ") lines the concave inner surface of the thick dorsal umbrella, and covers 

 its gelatinous body in the form of a thin, uniform, flat epithelium of very indifferent 

 character (PL IV. figs. 6-8, du; PL IX. figs. 5-7, da; PL XXV. figs. 8-10, du). The 

 ventral endoderm (or the " gastral epithelium of the coelumbrella," often also simply 

 termed the " subumbral or lower endoderm ") covers the convex inner surface of the thin 

 ventral umbrella, and is stretched across its subumbral supporting plate in the form of a 

 high differentiated cylindrical epithelium (PL V. figs. 6-8, div ; PL IX. figs. 5-7, dw ; 

 PL XXV. figs. 8-10, dw). Its cells are much larger than those of the dorsal endoderm, 

 are often extremely high, and enclose plasma products of various kinds, fat, granules of 

 pigment, crystals, amyloid granules, and other products of a vital change of tissue, but 

 also numerous vacuoles which not unfrequently coalesce. In many places, that is at 

 the oesophagus, one part of these ventral endoderm cells is transformed into glandular 

 cells, and another into urticating cells ; epithelial muscular cells and even perhaps sense 

 cells appear to originate from it here in some places. Finally, it is also these ventral 

 endodermal cells which form the reproductive cells in all Acraspedse ; both ova cells and 

 sperm cells proceed from a subepithelial layer of the ventral endoderm. This ventral 

 gastral epithelium is plainly of the highest significance for the aggregate changes of tissue 

 of the Medusas, whilst the opposite indifferent dorsal endoderm is only of slight importance ; 

 these cells are, moreover, " flagellate cells," as in both cases they invariably bear a vibrat- 

 ing flagellum. This flagellum is only missing on the cathammata, those important points 

 at which the dorsal and the ventral endoderm are fused together. Whilst the whole 

 gastral cavity originally shows in the polyps a perfectly simple cup-shaped cavity with- 

 out radial sections, in the Medusas it is divided in the course of development into peri- 

 pheric radial sections, by the fusion of the two walls of the gastral cavity (the dorsal outer 

 wall and ventral inner wall) in definite radia. In this way there originate the important 

 fused plates or cathammata which represent the septa of the radial chambers. Each 

 " cathamma " or " septum," therefore, actually consists of two layers of the gastral 

 endodermal epithelium which have been laid firmly one upon the other, and fused 

 together at those points. These two closely connected layers can sometimes be 

 plainly distinguished (as in many Acraspedse, PL XXV. figs. 8-10), and are sometimes 

 fused into a single simple layer (as in most Craspedotas). In both cases we designate 

 this simple or double layer of cells as the fused plate, or cathamma plate (" lamina 



