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



pierced by eight openings which conduct into the eight primary radial vessels, arising 

 from the original manubrium of the Medusa. The thick wall of the central siphon 

 exhibits eight strong radial longitudinal folds, and its slender proboscis has eight lips 

 around the terminal mouth (figs. 3, 5, so). 



Gonostyles (figs. 2, 3, 5, gs). — The sexual siphons, sixteen in number, form an 

 elliptical corona around the central siphon and separate it from the tentacular zone and 

 the mantle-border. Their slender cylindrical basal part bears clusters of medusiform 

 gonophores, whilst their dilated spindle-shaped distal part opens by a four-lobed mouth. 



Tentacles (figs. 1-5, t). — The sixteen tentacles form an outer corona around the inner 

 corona of gonostyles, and alternate regularly with the insertions of the latter. They are 

 simple, cylindrical, very contractile, and beset with two opposite rows of cnidoblasts. 



Genus 8. Velella, 1 Lamarck, 1816. 

 Velella, Lmk., Hist. nat. anim. sans vert., t. ii. p. 481. 



Definition. — Velellidse with an elliptical or slightly quadrangular (parallelogram - 

 shaped) umbrella, and a vertical triangular sail, placed obliquely in a diagonal line of the 

 ellipse. Margin of the umbrella not lobate. Pneumatocyst discoidal, of the same form 

 as the surrounding umbrella, with a vertical chitinous crest supporting the sail. Sub- 

 marginal corona of tentacles simple. 



The genus Velella is, next to Physalia, the oldest and best known form of all 

 Siphonophorse, since it is generally distributed in all warmer seas, often very common, 

 and in some countries used as food. Owina; to its striking form and sailing movement it 

 is well known to the fishermen. The Italian naturalists Ferrante Imperato (1599) and 

 Columna (1616) mention it under the names Vela or Velella, Carburius (1757) and 

 Dana (1776) under the names Armenista or Armenistarion. The first good description 

 and figure were given in 1776 by the excellent Swedish naturalist Forskal; 2 he named the 

 Mediterranean species Holothuria spirans. Afterwards Lamarck erected the genus 

 Velella with three species (the Mediterranean Velella limhosa, the North Atlantic Velella 

 mutica, and the South Atlantic Velella scaphidia). Esckscholtz 3 in his fundamental work 

 described ten different species, and Lesson 4 as many as sixteen. But the greater part of 

 these descriptions are worthless and quite insufficient, since they are founded only upon 

 the different coloration and other characters of little value (for example, the different 

 direction of the sail). 



Two species of Velella only are hitherto completely known ; the Mediterranean 

 Velella spirans, very accurately described by Kolliker 5 and by Vogt, 6 and the North 

 Atlantic Velella mutica by Alexander Agassiz. 7 



1 Velella = Diminutive of Vela, sailing-boat. 



2 11, Taf. xxvi. fig. K. 3 ^ p 168j Ta f. xv. 4 3, p. 563. 



6 4, p. 46, Taf. xi. figs. 9-15. u 6, pis. i., ii. 7 57, pis. i.-vi. 



