THE HYDROMEDUSAE 



FIG. lOfo. Scattered nerve -ganglion cells from the 

 subumbrella of Aurelia. (From Lankester, after 



tentacles. Subepithelial ganglion cells and nerve fibrillae form a 



scattered plexus in the ectoderm in connection with sensory and 



muscle cells, especially on 



the subumbrella ; they are 



concentrated at the lip of 



the bell into a nerve ring, 



which is divided by the 



insertion of the velum into 



outer and inner portions, 



connected by nerve fibrils 



through the mesogloea. 



Connected with the 

 nerve ring are the sense 

 organs (Kertwig, 1 9 ; Eimer, 

 20) or special aggregations 

 of sense cells. Thev are 



referable to four chief 

 types. 



1 Ocelli nr PVP snnfs subumbri 



1. uceui eye spots Scll jif er .) 



are generally found at the 



bases of tentacles. In their simplest form they consist of a few 

 sense cells between which are scattered a few pigment cells ; in their 

 complete development, the sensory and pigment cells are grouped 

 into a definite organ of subspherical shape (Fig. 11), which projects 

 above the general surface, and may secrete a cuticular lens (Lizzia). 

 The whole structure is ectodermal. 



2. Otoci/sts are found under two chief forms : (a) in the simpler 

 of these the organ consists of an open subumbral pit at the base 

 of the velum, the cells of which secrete each an otolith of organic 

 and calcareous nature (Mitrocoma) ; (b) in the more complex type 

 the pit becomes converted into a closed vesicle, containing one or 

 more otolithic cells, which are usually supported on sensory hairs. 

 The whole structure is ectodermal, and may occur either on or 

 between tentacles (Figs. 12, 13). 



3. Cordyli (Brooks, 21) are exumbral structures, placed between 

 tentacles, which consist of a core of vacuolated endoderm cells 

 covered by flattened ectoderm. It is possible that they represent 

 a modification of a tentacle, less complete than, but analogous to, 

 the modification which has produced the next form of Hydro- 

 medusan sense organ (Fig. 15). 



4. Tentaculocysts, which are apparently tentacles modified for 

 the better perception of auditory vibrations, and are placed exum- 

 brally, consist essentially of a club-shaped structure, clothed extern- 

 ally by ectoderm ; they contain an axial core of endoderm cells, the 

 outermost (one or more) of which secretes an otolith. The club 

 thus formed either projects freely from an eminence composed of 



