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



ectoderm in one order only (the Leptomedusaj) and in all other orders to the endoderm. 

 In some cases {e.g., in most Leptomedusse and in the Geryonidse) the auditory cells 

 become completely separated from the free epithelium to which they originally belonged ; 

 they are then transformed into mesodermal interior epithelium, as the open olfactory 

 depressions become detached from the dermal covering and from closed auditory 

 vesicles. 



§ 56. Ganglion cells or mesodermal nerve cells (" celluke gangliosse," " neuroblast! "). 

 The ganglion cells bear the same relation to the sense cells as the mesodermal muscular 

 cells do to the epithelial. The ganglion cells are, in fact, subepithelial nerve cells 

 secreted from the epithelium, from which they have originated both ontogenetically and 

 phylogenetically ; they are still connected directly or indirectly with this their point 

 of origin by thread-shaped processes, the nerve-fibrillae (PI. XIV. fig. 10). All gang 

 lion cells of the Medusae appear to have two or more processes, and are, therefore, 

 either bipolar (fusiform) or multipolar (stellate) cells. Both forms appear both in the 

 central and in the peripheric nervous system ; the bipolar cells, however, preponderating 

 in the central nerve ring, the multipolar ganglion cells in the peripheric nervous 

 plexus ; the former therefore lie principally in the umbrella margin, the latter in the 

 subumbrella. The central ganglion cells, moreover, both in the nerve ring and in the 

 organs of sense, show definite conditions of relation and position to the neighbour- 

 ing organs, especially to the sense cells of the epithelium. In the Craspedotse, the dorsal 

 (or exumbral) nerve ring covered by the sense epithelium is formed for the most part 

 of parallel lying, circular fibrillae, and is much poorer in ganglion cells than the ventral 

 (or subumbral) nerve ring which has no sense epithelium and is more motor. In the 

 Acraspedae the ganglion cells seem rather to be accumulated at the bases of the sense 

 clubs, and to form four or eight ganglia which are sometimes connected by a centralised 

 ring of bundles of fibrillse (Cubomedusae), sometimes by a more decentralised plexus 

 of fibrillae. The peripheric ganglion cells are scattered, sometimes sparsely, sometimes 

 pretty numerously in the nervous plexus, which extends chiefly in the subumbrella in 

 the form of delicate,reticulate plexus of fibrillae ; this subumbral plexus lies between 

 the muscular plate of the subumbrella and the endodermal epithelium, from which the 

 latter has arisen. Both this peripheric nerve plexus and the central nerve ring may 

 already be regarded as mesodermal nerves, as they possess independent cells, secreted 

 from the epithelium. 



