liv 



THE VOYAGE OF H.M.S. CHALLENGER. 



all the Acraspedse (with the single exception of the Stauromedusse) and in the majority 

 of the Craspedotse, in all Trachomedusse and Narcomedusae, and also in the Vesiculatse 

 (Eucopida? and ^Equoridaa) among the Leptomedusse. In the Vesiculatse, however, the 

 organs of hearing have quite a different structure and a different origin from those in 

 the other groups mentioned, the Vesiculatee have velar auditory vesicles with ectodermal 

 otolites, whilst all the others have tentacular auditory clubs with endodermal otolites. 

 These two types in the structure of the auditory organs differ so entirely that they 

 require a separate description. 



§ 83. Velar auditory vesicles (with ectodermal otolites, " vesiculae velares," also 

 termed " marginal vesicles," " vesiculae marginales," ov). This peculiar form of the audi- 



Fig. E. Eucopc campanulata (Leptomedusre, Eueopidse). 



Subumbral aspect, («) Quadrate oral opening, (s) Ovaries, (cp) Perradial canals, (cc) Coronal canals. 

 (v) Velum, (ov) Velar marginal vesicles (adradial). (7s) Umbrella cavity, (re) Nerve ring, (oc) 

 Ocelli at the swollen bases of the tentacles, (ug) Gelatinous substance of the umbrella. {I) Tentacles. 



tory organs is found exclusively in the order of the Leptomedusae, and there only in a 

 suborder which we name Vesiculates for this reason (the two families, rich in forms, of 

 the Eucopidse and iEquoridae, System, taf. x.-xv. pp. 116, 165, 210). The other sub- 

 order of the Leptomedusse, the Ocellatse (the two families of the Thaunianthidae and 

 Cannotidse) have neither otolites nor auditory vesicles, but perhaps auditory cells on the 

 subumbral side of the velum. This exception is supported by the fact, that in the simplest 

 cases in the Vesiculatse (Mitrocoma) no closed auditory vesicles are present, but only open 

 depressions in the insertion of the velum, in whose ectoderm auditory cells and otolite 



