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



ing the epithelium at the distal end of the tentacle canal. The auditory clubs of the 

 Acraspedse are, moreover, combined in a peculiar way, with other organs of sense (ocelli, 

 olfactory depressions, tactile plates) and with their surroundings, compose the typical 

 sense clubs or rhopalia, which we shall afterwards consider separately. On the other 

 hand the analogous auditory clubs of the Craspedotse (of the Trachomedusae and Narco- 

 medusae) which often closely resemble the others, appear to be more simple formations 

 (Pis. III. -XI V., oh). They resemble perfectly simple, sobd tentacles, whose axis consists 

 of a row of a few endoderm cells (usually two to four, more rarely five to ten or more), 

 processes of the inner epithebum of the coronal canal. Either only the last of these or 

 several of them (two to four, rarely more) produce in their interior a calcareous concre- 

 ment, which functions as an otolite. The Trachomedusae (and a small part of the 

 Narcomedusae, the Solmaridse) have only a single round otolite in each auditory club ; 

 it is in concentric layers, usually spheroidal, more rarely elliptical, and often coloured 

 red or yellow. Most Narcomedusae (all, indeed, except the Solmaridae) possess crystalline 

 otontes of prismatic form (usually several in each auditory club). The acoustic ectoderm 

 epithelium of the auditory clubs is separated from the sobd endoderm axis by a thin 

 supporting plate, and bears long stiff auditory hairs ; so is the ectoderm of the " auditory 

 pad " or " auditory papilla," which in many Narcomedusae arises at the base of the 

 auditory club by a swelling of the dorsal nerve ring ; the latter always supphes the 

 cordylus. In part of the Narcomedusae (the Cunanthidae and Peganthidae) peculiar, firm 

 urticating streaks are found at the bases of the cordyli which rise from these centripetally 

 into the exumbrella, and are covered with ciliated sense epithelium (auditory clasps, 

 " otoporpae," PI. IX. fig. 8, oo ; PI. XL fig. 4, 06). Four interradial auditory clubs seem 

 usually present originally ; the number often increases largely later on, and may amount 

 to more than a thousand (e.g., PegantJm magnified, System, p. 333). 



§ 85. Cordylar auditory vesicles ("vesiculae cordylares"). Whilst in all Narcomedusae, 

 and also in the lower and older groups of the Trachomedusae (Petasidee, Pectyllidae, Pis. 

 III. -VIII., Aglauridae), the auditory clubs stand freely on the umbrella margin, in 

 some of the younger and higher groups of the Trachomedusae, this is rarely the case, 

 and then only in the young stage. The originally free auditory clubs become en- 

 closed in special " auditory vesicles." In the Marmanemidae (System, taf. xvii.) this 

 is caused by the ectodermal epithelium of the dorsal nerve ring rising like a wall, in the 

 form of a circular fold, at the base of the free cordylus ; its margins grow together above 

 the depression formed, and so transform it into a closed vesicle ; the auditory hairs are 

 stretched like harp strings between the inner wall of the vesicle and the upper surface of 

 the cordylus enclosed. Whilst these " auditory vesicles " of the Marmanemidae lie freely 

 on the umbrella margin, the similarly constructed auditory vesicles of the Geryonidae lie 

 deeply inserted in the gelatinous body of the umbrella margin. These cordylar auditory 

 vesicles of the Marmanemidae and Geryonidae therefore differ entirely both in origin and 



