340 MEDUSAE OF THE WORLD. 



Many of the Trachymedusae, being independent of an attached hydroid stage, are widely 

 distributed over the open oceans, being merely confined within more or less definitely defined 

 ranges of latitude and extending within these limits around the world. The species are far 

 more abundant in tropical or warm seas than in polar regions. Many of them are creatures 

 of that which Bigelow (1909, Mem. Museum of Comparative Zool. at Harvard College, 

 vol. 37, pp. 221-235) has aptly termed the "intermediate fauna" ; that is to say they rarely 

 or never approach the surface, being probably negatively phototoctic to light of any consider- 

 able intensity, and they live at depths ranging from several hundred fathoms to greater 

 depths, or even to the bottom. Bigelow finds that great numbers of Trachymedusae and 

 other pelagic forms are found in the cold Humboldt, or Chili-Peruvian current, off the 

 western coast of South America. These creatures are, moreover, confined quite strictly to 

 the current, and on its western edge one suddenly enters upon a vast oceanic area nearly 

 devoid of life. I have often observed this sudden transformation from a region rich in lite 

 to one barren of organisms on the edge of the Gulf Stream, off the Florida coast. Other 

 factors beside those of light and temperature may also contribute to force these medusae to 

 remain at considerable depths beneath the sea, for I have frequently observed that they are 

 more abundant near the surface when the ocean has been perfectly calm for a few days than 

 under ordinary conditions ; for if they are undisturbed by the movement of the waves some 

 of these forms, in common with many of the rarely seen species of Ctenophorae and Siphon- 

 ophoras, will gradually rise upward until they become visible in great numbers over the 

 unrippled surface. 



Owing probably to their great range in distribution we find many local races or closely 

 related varieties of Trachymedusae which it is exceedingly difficult to separate from their near 

 relatives, and great confusion has been introduced into the synonymy of such forms. This is 

 especially noteworthy in such genera as Aglaura, Homceonema, Liriope, and Geryonia. 



We may define the families of the Trachymedusae as follows: 



OVmdiadce Haeckel, 1879. Some or a11 of thc t™ tacles project from the sides of the exumbrella above the margin and have 



adhesive disks. Gonads linear, folded, or sac-like, and developed upon the 4 or 6 radial-canals. 4 lips. 

 Subfamily Petasidce, Haeckel, 1879. Tentacles without adhesive disks. In other respects this family resembles the very 



closely allied Olindiad*. 

 Limnocnidid*. Tentacles hollow, without adhesive disks, and projecting from the sides of the exumbrella above the bell-margin. 



Sexual products diffusely developed over the sides of the stomach. 4 to 6 radial-canals. Mouth a round opening. No 



centripetal canals. 

 Ptychogastrid<r. Some filiform and some sucker-bearing tentacles, grouped in clusters. Gonads upon the sides of the 8 lobes of 



the stomach. 8 radial-canals. 4 quadratic lips. Centripetal canals present. 

 Trachynemid* Gegenbai>r, 1856. Tentacles solid, filiform and without adhesive disks. Linear or sac-like gonads on the 8 



radial-canals. 4 lips. With or without a peduncle. 

 Geryonidx Eschscholtz, 1829. Three sets of dissimilar tentacles. The gonads are flat, leaf-like expansions upon the 4 or 6 



radial-canals. 4 or 6 lips. Stomach mounted upon a gelatinous peduncle. 



Family OLINDIAD^, sensu Goto. 



Olindiadx, Haeckel, 1879, Syst. der Medusen, p. 252.— Goto, 1903, Mark Anniversary Volume, pp. 15, 19. — Mayer, 1904, 



Mem. Nat. Sci. Museum Brooklyn Inst. Arts and Sci., vol. I, pp. 18, 21. 

 OVmdiadce (in part), Browne, 1904, Fauna and Geog. Maldive and Laccadive Archipel., vol. 2, part 3, p. 736; 1905, Report 



Pearl Oyster Fisheries, Gulf of Manaar, Suppl. Report No. 27, Roy. Soc. London, p. 150. 



Perkins and Murbach conclude that the marginal concretions of Gonionemus are of ento- 

 dermal origin and that this genus is one of the Trachymedusae, while Goto, who is an exceed- 

 ingly careful observer, concludes that the concretions of Olindioides are of ectodermal origin 

 and that the medusa belongs to the Eucopidae. 



FAMILY CHARACTERS. 



Veiled medusae, in which some or all of the tentacles project from the sides of the 

 exumbrella above the margin, while others may arise from the bell-margin. Some or all of the 

 tentacles are provided with adhesive pads bearing a superficial resemblance, to suctorial disks. 

 4 or 6 linear, sac-like, or papilliform gonads upon the 4 or 6 complete radial-canals. Blindly 

 ending centripetal diverticula from the ring-canal may or may not be present. The lithocysts 

 may project freely below the bell-margin or be inclosed within the gelatinous substance of the 

 bell above the ring-canal. 



