REPORT ON THE DEEP-SEA MEDUSAE. XXXV 



important and widely diffused ectodermal nervous tissue are most closely connected with 

 the epithelium from which they are originated. The whole muscular system in the 

 Medusse preserves more or less its original epithelial character. We can therefore 

 distinguish here, as in the muscular tissue, purely epithelial and subepithelial cells ; the 

 former may usually be termed sense cells, the latter ganglion cells. Both are connected 

 by very fine fibrillse, thread-shaped processes, which are repeatedly branched and com- 

 bined into network and nerve plexus. 



§ 55. Sense cells or epithelial nerve cells ("cellulse sensillares," "sensoblasti"). Under 

 this title we include all these nerve cells (in the widest sense) which lie in the 

 epithelium itself, and which have fine thread-shaped processes or fibrillar at their bases, 

 by which they are connected with other cells of the nervous system. These sense cells are 

 sometimes scattered singly in the epithelium between its indifferent covering cells {e.g., 

 on many places in the tentacles and oesophagus), sometimes, as a connected covering, 

 they form a true sensitive epithelium {e.g., on the dorsal nerve ring of the Craspedotae, 

 and on the margin of the velarium of the Acraspedae). We can distinguish two 

 principal forms of sense cells, indifferent or neutral, and differentiated or specific sense 

 cells. We call those epithelial sense cells " indifferent," to which we cannot assign a 

 specific function of sense, and which therefore represent the oldest and simplest form of 

 the nervous elements. Provisionally we may consider as such in the Medusas, all those 

 flagellate cells and bristle cells of the ectoderm which are connected at their base, 

 directly or indirectly, by processes and nerve fibrillar with other nerve cells, and in which 

 we recognise no specific sensitive function (PI. XIV. fig. 9). All these neutral sense cells 

 have a fine hair-shaped process on their free upper surface, which is movable in the 

 sensitive flagellate cells {e.g., on the sense epithelium of the nerve ring), but stiff in the 

 bristle cells. We include in the latter the true tactile cells (without nematocysts, with a 

 tactile bristle, " palpocilium ") and also the thread cells (with nematocysts and with an 

 urticating bristle, " cnidocilium "). How far the flagellate cells and thread cells of the 

 ectoderm belong to the category of indifferent sense cells depends upon their passing at 

 their base into communicating processes or nerve fibrilke (comp. below § 79, organs of 

 touch). We may consider as differentiated or specific sense cells such epithelial nerve 

 cells to which from their situation, structure, or connection, we can assign some specific 

 sensitive function. To this category belong — (l) the olfactory cells (or gustatory cells ?) 

 on the clavelhe of the Craspedotas (PI. II. fig. 8, q) and on the protective scales of the 

 rhopalia in the Acraspedse ; (2) the vision cells of the eyes, which are sometimes 

 differentiated into pigment cells and rod cells ; (3) the auditory cells of the auditory 

 organs (PL VI. fig. 16). The last bear a free, usually long and thin auditory hair, and 

 therefore do not differ in form from the ordinary tactile cells (with a tactile bristle) from 

 which they are also derived phylogenetically ; they become " auditory cells" as the)" are 

 in functional connection with " otolite cells " which contain an otolite, and belong to the 



