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



Central Siphon (fig. 1, sm, fig. 4). — The large sterile central polypite is inversely 

 conical, in the contracted state about as long as the diameter of its broad base, which is 

 a quarter or a fifth of the diameter of the umbrella. Its thick muscular wall exhibits 

 sixteen longitudinal folds, and between these run sixteen deep grooves, which open at the 

 bottom of the stomach into the sixteen radial main canals. The distal mouth of the 

 central siphon is octolobate. Its opposite proximal roof is separated by a strong fulcrum 

 or supporting lamella, the gastrobasal plate (fig. 1, st), from the superjacent kidney. 



The sixteen radial main canals, which arise from the periphery of the base of the 

 stomach, run along the subumbrella towards the peripheral edge of the limb, where they 

 are united in a circular marginal canal. They give off innummerable branches ; of these 

 the ascending ones enter into the centradenia, the descending partly into the gonostyles, 

 partly into the tentacles. Numerous superficial mantle- vessels arise from the marginal 

 canal and enter centripetally into the exumbrella, where they form an irregular dense 

 network above the pneumatocyst. 



Sexual Si£)hons (figs. 1,8, sx). — The gonostyles cover the greatest part of the sub- 

 umbrella, and are densely crowded in the broad zone which lies between the central siphon 

 and the tentacular zone. Their number amounts to several hundreds. They are covered 

 with bunches of gonophores in the cylindrical proximal half, with patches of cnidocysts in 

 the spindle-shaped distal half. Their distal mouth is often divided into four or eight 

 small lobes. 



Tentacles (fig. 1, t). — The tentacular zone of the subumbrella, inside the free pro- 

 minent limb, is about as broad as the base of the central siphon. It presents an elegant 

 reticulum, with rhomboidal meshes, when the tentacles are removed ; each mesh being 

 the base of a detached tentacle. These are arranged in six to eight concentric rows. In 

 the largest specimens their number exceeds one thousand. Their length may exceed the 

 diameter of the umbrella ; the majority, however, are mueh shorter. The structure of the 

 club-shaped tentacles is the usual one, with three rows of stalked cnidospheres (compare 

 p. 39). 



Family III. Velellid,e, Eschscholtz, 1 829. 



VeleUidx, Eschscholtz, System der Aealephen, p. 165 (sensu restricto). 



Definition. — Disconectse with an elliptical or amphithect, often nearly quadrangular 

 umbrella, including a polythalamous pneumatocyst of the same form, which is composed 

 of numerous concentric rings, and usually bears in its diagonal a vertical crest. Always 

 a membranous vertical sail upon the umbrella. Marginal tentacles simple, without cnido- 

 spheres. Central siphon surrounded by numerous peripheral fertile siphons, which bear 

 the gonophores. 



The family Velellidae, founded by Eschscholtz in 1829, comprised originally all Dis- 



