HYDROMEDUSAE, SIPHONOPHORES, AND CTENOPHORES. 321 



There are 12 solid tentacles — 6 radial, 6 interradial — arising, of 

 course, from the margin, but curving upward against the bell, in 

 open furrows of the exumbrella, to emerge a slight distance above the 

 margin. The two (both interradials) which are intact in the type- 

 specimen are smooth walled for the inner four-fifths of their length, 

 but the ends are ringed with about 20 nematocyst ridges, with a large 

 nematocyst knob at the tip. 



There is an otocyst close beside each tentacle root, within the ex- 

 umbral furrow (pi. 43, fig. 5), but standing free, not inclosed by the 

 gelatinous substance. The organs themselves are naked, spherical 

 in form, consisting of an ectodermic covering layer, inclosing an ento- 

 dermic core with about four large spherical cells, each containing a 

 central mass, the high index of refraction of which shows that it is 

 an otolith (pi. 43, fig. 3). 



The discovery of free sense-clubs in Nauarchus is especially inter- 

 esting because up to this time the only records for sense-organs of 

 this type in the Petasidae are Haeckel's accounts of his problematic 

 genera Petasus, Dipetasus, Petasata, and Petachnum. In all the 

 other genera of the family, as Gonionemus, Olindias, Eperetmus, 

 Aglauropsis, Cubaia, Vallentinia, Olindioides, Craspedacusta, and 

 probably in Gossea, the sense-organs lie in vesicles, which themselves 

 are usually inclosed in the mesogloea. 



The otoliths of Nauarchus are clearly entodermal, just as in the 

 Trachynemidae ; and, according to Perkins and Murbach, this is also 

 true of Gonionemus, as it apparently is of Eperetmus (19155, pi. 59, 

 fig. 8). That is to say, the sense-club of Nauarchus, or indeed that 

 of any Trachomedusa in which this type of sense-organ occurs, cor- 

 responds essentially to the strand of cells with terminal concretion, 

 which lies within the vesicle of Gonionemus. Both are modified ten- 

 tacles. And although Goto (1903) believed the otocysts of Olindias 

 to be of ectodermal origin, it is not likely that they are fundamentally 

 different in that genus from in its allies. Rhopalonema, in which the 

 club, at first free, is later enclosed by the upgrowth of a crater-like 

 vesicle, may epitomize the relationship here outlined. 



Family TRACHYNEMIDAE Gegenbaur, 1856. 



Genus COLOBONEMA Vanhoffen, 1902. 



Colobonema Maas (1905), Browne and Fowleb (1906), Bigelow (1909a). 

 Bomoeonema Part Mayer (1910). 



Trachynemidae, with tentacles all of one kind, 32 in number, of 

 which the 8 perradial, the 16 adradial, and finally the 8 interradial 

 develop in succession. 



