6 THE VOYAGE OF H.M.S. CHALLENGED. 



Sub-family, Polyorchid.e, A. Agass., 1862. 



Cannotidae with four or six radial canals, which are pinnated, or furnished with 

 csecal side branches which do not reach into the circular canal. 



Ptychogena, 1 A. Agass., 1865. 



Cannotidaa with four pinnated radial canals, whose alternating pinnated branches 

 bear several leaf-shaped cleft, indented or compound genitalia. Stomach a flat, wide 

 pouch, without special oral lobes. 



The genus Ptychogena was established by A. Agassiz, in 1865, for the North 

 American deep-sea form, Ptychogena lactea (North American Acalephae, p. 137, 

 fig. 220). A second somewhat different deep-sea species from the North- Atlantic Ocean 

 {Ptychogena pinnulata) is here described, and completes Agassiz's short definition. 

 Ptychogena is the connecting fink between the apparently very different genera Gony- 

 nema and Staurophora. Whilst the stomach is a long tube in Gonynema, and is entirely 

 rudimentary in Staurophora, in Ptychogena it forms a flat, wide-opened quadrate pouch, 

 whose four corners pass conically contracted without definite limits into the four radial 

 canals ; and whilst in Gonynema the pinnated branches of the genitalia are entirely 

 limited to the radial canals, but in Staurophora run centripetally to the centrum of 

 the cross of these canals, they are developed in an intermediate degree in Ptychogena. 

 They there occupy only the proximal half of the radial canal, but pass from it some 

 distance upon the wall of the stomach. Both North Atlantic species of Ptycho- 

 gena appear to be true deep-sea Medusae. A. Agassiz writes of it as follows (loc. cit., 

 p. 139):— 



" This Medusa, like Tima, swims at a considerable depth below the surface. The action 

 of the light and increase of temperature of the surface is sufficient to kill them in the 

 course of half an hour ; the moment they are brought to the surface the spherosome loses 

 its transparency, the genital organs become dull, and the Medusa is soon completely de- 

 composed. This action is much more rapid than anything I have noticed even in Cteno- 

 phoras, Mertensia being the only genus in which the decomposing effects of light and heat 

 are at all equal to what is produced here. This Jellyfish must be a deep-water species, as 

 they have only been found during a single fall, and then only for a few days, when they 

 seemed quite abundant." 



These remarks most probably are applicable to Ptychogena pinnulata as well as 

 Ptychogena lactea. The example of the former in the Challenger collection was found 

 at a depth of 1250 fathoms. 



1 IIti/xi), turning ; y-vri, reproductive organs. 



