1902.] NATURAL SCIENCES OF THILADELPHIA. 787 



form, loug and bean-shaped (PI. XXXIV, fig. 27). Examina- 

 tion with a high-power objective, focussing down into the w^ater 

 upon the extended tentacle, shows with considerable distinctness a 

 ganglion cell of glistening highly refractive appearance, lying close 

 to each nematocyst (PI. XXXIV, fig. 27, g.c). In every case this 

 ganglion cell is situated distal to the thread -capsule, toward the free 

 end of the tentacle (PI. XXXIV, fig. 26). A thin strand of ner- 

 vous tissue runs in each direction from the ganglion cell, towai'd the 

 nematocyst proximally, toward the free tip of the tentacle distally. 

 It is visible for only a short distance, however; soon vanishing into 

 the ectodermal tissue, and none of its branches or terminations are 

 to be traced. It evidently innervates the nettling capsule, near the 

 base of which it can be seen. 



In the gonosome the nematocysts are carried out onto the ecto- 

 derm of the growing tentacle in situ, as in the larva. Further 

 growth in the extent of the ectoderm is brought about in two ways : 

 By multiplication of the cells already incorporated in the epithelium 

 of the tentacle, and by immigration of the cells from the thick ecto- 

 dermal pad at the base. The tissue composing the pad is peculiar 

 in character. The cell-walls are almost or quite obliterated, and 

 the gelatinous substance contains the cell-products already men- 

 tioned. In this whole group of medusse the older tentacles are left 

 stranded, as it were, by the grov/th of the margin of the umbrella 

 beyond their point of origin. As they are in this way carried up 

 on to the exumbral surface of the medusa, the pad of ectodermal 

 tissue grows so as to fill the space between the base of the tentacle 

 and the bell-margin, forming a round cushion of hard tissue. In 

 sections cut through this tentacle-pad (PI. XXXIII, figs. 22 and 

 23) it is seen that the concretions which lie towai'd the bell-margin 

 are more dense and homogeneous ; that further inward they are 

 somewhat less solid in appearance, spaces appearing within their 

 outer walls; and that at the side nearest the circular canal there 

 are great numbers of nettling cells in various stages of formation. 

 All gradations are present between the solid concretion and the 

 nettling cell (PI. XXXIII, fig. 22). Fig. 22 was drawn by Prof. 

 Brooks to show this condition in Gonionema. At the inner margin 

 of the ectodermal pad the nematocysts lie closely packed together 

 (fig. 22). From this breeding place they work their way out on 

 to the tentacle along which they migrate until they reach a spot 



