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



All Porpitidae probably pass during their metamorphosis through a larval stage, 

 which is essentially identical with Discalia, and differs from it only in the absence of 

 sexual palpons or gonostyles, bearing gonophores. The young larva of Porpita linnseana, 

 which Alexander Agassiz has figured, 1 exhibits a larval stage which is a little more 

 advanced, and forms the transition to the Disconalia-stage. 



The phylogenetic importance of Discalia, therefore, is very great. On the one hand, 

 it seems to indicate clearly the origin of the Disconectse frorn the Trachomedusaj (Trachy- 

 nemida?) ; on the other hand, all the other Disconectse may be derived from it as from a 

 common ancestral genus. 



Two species of Discalia, both inhabitants of the deep sea, were found by me in the 

 Challenger collection. The first species, Discalia medusina, was taken in the centre of the 

 Southern Pacific, at Station 288 (depth, 2600 fathoms), and is figured in PI. XLIX. figs. 

 1-6. The second species, Discalia primordialis, was captured in the tropical Pacific, at 

 Station 274 (depth, 2750 fathoms). The latter is distinguished from the former by a more 

 flattened umbrella, and by eight longer (perradial) tentacles, between which eight shorter 

 (interradial) were interpolated ; but as its state of preservation was not sufficient, I give 

 here only the description of the first well-preserved species. 



Discalia medusina, n. sp. (PL XLIX. figs. 1-6). 



Habitat.— Southern Pacific, Station 288, October 21, 1875 ; lat. 40" 3' S., long. 

 132° 58' W.; depth, 2600 fathoms. 



Umbrella (fig. 1, view from below ; fig. 2, from above ; fig. 3, in profile ; fig. 4, in 

 meridional section). — The umbrella, which represents the nectosome, is subspherical, 

 0'2 to 0'4 mm. in diameter. A deep annular constriction separates the natter exumbrella 

 from the inflated and highly vaulted subumbrella. The latter bears in its equatorial 

 zone a corona of eight simple tentacles, and beyond it a corona of eight gonostyles, 

 which surround the central siphon. 



Exumbrella (fig. 2). — The superior or apical face of the umbrella is rather flat, and 

 exhibits an octoradial star of brown pigment, indicating the course of the eight per- 

 radial exumbrellar canals, or the superior vessels of the mantle. The apical stigma, or the 

 superior opening of the central air-chamber, is situated in the centre of the pigment- 

 star, whilst eight other stigmata, the outer openings of the eight interradial air- 

 chambers, are placed between the eight rays of the dark pigment-star. 



Limbus Umbrella} (uu). — The circular margin of the umbrella is thickened and divided 

 by eight perradial incisions into eight prominent flat interradial lobes. The whole 

 margin is beset with a corona of the usual marginal glands (fig. 6) (compare p. 42 above). 



1 57, pi. ix. figs, l, 2. 



