REPORT ON THE DEEP-SEA MEDUSAE. lv 



cells are developed. These auditory depressions (" fossulae velares ") are probably found 

 in other Leptomedusae (Phialis, Tiaropsis, Mitrocomella, &c), besides Mitrocoma 

 (System, taf. x.). They form small depressions in the subumbral or ventral side of the 

 velum (which is commonly termed the " lower " side but which is the " upper " in the 

 normal position of the horizontally stretched velum). One portion' of the subumbral 

 ectoderm cells, which line these depressions, and which are connected with the contiguous 

 ventral nerve ring, forms a calcareous otolite in their interior, another portion of it bears 

 an auditory bristle. As these " auditory depressions " become deeper, enter the dorsal 

 side of the velum vaulted like an arch, and finally become entirely loosed from the ventral 

 side, they are transformed into auditory vesicles ("vesicuke velares"). These project 

 more or less as conical or roundish vesicles on the dorsal side of the velum, near its inser- 

 tion on the umbrella margin, become innervated from the subumbral nerve ring, covered 

 externally by the dorsal ectodermal epithelium of the velum and contain a hollow space 

 filled with otolymph (originally sea water) ; this space is lined with an acoustic 

 epithelium, which originally belongs to the ventral ectodermal epithelium of the velum 

 and consists partly of auditory cells bearing bristles and partly of otolite cells. The 

 auditory hairs of the former surround or He upon the latter. In the most simple cases, 

 each velar marginal vesicle only contains a single otobte, but in others often a large number 

 of them. The inner (subumbral) sense epithelium and the outer (exumbral) covering 

 epithelium, are separated by a structureless lamella which belongs to the supporting 

 plate of the velum. The number and distribution of these velar auditory vesicles of the 

 Leptomedusae varies largely ; however, there are always originally eight adradial audi- 

 tory vesicles, which lie exactly in the middle between the four perradial and the four 

 interradial tentacles (fig. E). We never find fewer than eight. In most Leptomedusae 

 their number is considerably increased, often to several hundreds ; we may therefore dis- 

 tinguish two groups of the Vesiculatae, the Octotessae, and Polyotessae, the former having 

 invariable eight velar auditory vesicles, the latter having invariably more than eight 

 (System, p. 117, taf. x.-xv.). 



§ 84. Auditory clubs or " cordyli " (ok), acoustic tentacles with endodermal otolites. 

 This form of the organs of hearing is by far the most common among the Medusae, and is 

 found in the majority of the class, in the Trachomedusae and Narcoruedusae among the 

 Craspedotaa, and also among all the Acraspedae, with the single exception of the Stauro- 

 medusae. The auditory clubs of all these Medusae are modified, small acoustic tentacles, 

 containing endodermal otolites, and differing therefore entirely in origin and composition 

 from the velar auditory vesicles of the Leptomedusaa (with ectodermal otolites). In the 

 two sections, the auditory clubs have originated, independently of one another, from the 

 tentacles in an analogous manner, from the solid tentacles in the Craspedotae, from the 

 hollow tentacles in the Acraspedae. In the former, it is the chordal cells of the solid 

 endoderm axis, which produce the otolites, in the latter it is the endodermal cells form- 



