60 THE VOYAGE OF H.M.S. CHALLENGED 



eight meridional arches, and open on the periphery of its inferior polar area into the gastral 

 cavity of the central siphon. Their dichotomous branches run along the convex outer 

 surface of the centradenia, enclosed in the numerous radial folds which fit into the 

 corresponding meridional furrows of the subumbrellar face of the pneumatocyst. 



Central Siphon (fig. 2, in profile ; fig. 3, in longitudinal section). — The large central 

 polypite is a thick-walled cylindrical tube, the length of which surpasses the vertical main 

 axis of the umbrella. Its distal end opens by a four-lobed mouth, whilst its proximal or 

 basal part is conically dilated and closed by the supporting plate separating it from the 

 centradenia (fig. 3). In the periphery of the latter open the eight radial main canals. 



Sexual Siphons (figs. 2, 3, sx). — A corona of eight sexual polypites surrounds the base 

 of the central siphon, and separates it from the tentacular zone. These are much smaller 

 than the central siphon (about half as long, and many times thinner). Their thin cylin- 

 drical basal part is beset with numerous gonophores ; their spindle-shaped distal part 

 opens by a four-lobed mouth. 



Tentacles (figs. 1-3, t). — The tentacles are very numerous, and occupy a broad convex 

 zone of the subumbrella, between its ecpuator and the girdle of sexual siphons. After 

 removal of the tentacles, their insertion forms an elegant reticulate girdle, with rhom- 

 boidal meshes (fig. 2, tu). The tentacles are arranged in four to five transverse rows, 

 and in eight prominent radial bunches ; the longest of each bunch surpassing the diameter 

 of the umbrella. 



Genus 4. Porpema, 1 Haeckel, 1888. 



Porpema, HkL, System der Siphonophoren, p. 30. 



Definition. — Porpitidae with a lenticular or subglobular strongly vaulted umbrella, 

 including a campanulate pneumatocyst with radial marginal lobes. Tentacles very 

 numerous, equally disposed in several concentric circles, not forming radial bunches. 



The genus Porpema has the same strongly vaulted umbrella and the same campanu- 

 late and radially lobate pneumatocyst as the preceding genus Porpalia, from which it 

 differs in the equal distribution of the tentacles along the whole margin of the umbrella. 

 The tentacles are very numerous and densely crowded in several parallel circles (as in 

 Porpita), and they are not grouped into radial bunches. 



The new genus Porpema was established for an Indian species, Porpema lenticula, 

 which I observed in 1881 in the Indian Ocean, between Aden and Bombay. Another 

 species, described in the following pages as Porpema medusa (PI. XL VII.), was found in 

 the Challenger collection ; this was taken in the South Atlantic, Station 327 (between 

 Buenos Ayres and Tristan da Cunha). A third species (Porpema pileata) was sent me 

 from Chili ; it will be described in my Morphology of the Siphonophorse. 



1 Porpema = Mantle, cloth, ■jricvnft.a. 



