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



lateral and two weaker medial,' an upper and a lower). The cnidoblasts are often more 

 crowded towards the distal end. Sometimes they are irregularly scattered. 



Ontogeny. — The larvae of the Velellidae were described by Eschscholtz, 1 who founded 

 upon them the new genus Rataria. We retain here this genus for those simplest 

 forms of the family, which, at this same stage, develop a corona of sexual siphons around 

 the central siphon, and gonophores from the gastral walls of these gonostyles. The 

 larval forms of Velella, described usually as Rataria, lack the gonostyles, and are better 

 designated as Ratarula. Pagenstecher (55) has given a careful historical and anatomical 

 description of these larval forms. But I suppose that the numerous larvae examined by 

 him, and all figured as Rataria, may be mixed larvae of Velella and Porpita. The 

 regular octoradial pneumatocyst, figured in his Taf. xli. fig. 1 , seems to belong to Porpita, 

 since the same organ in Velella is always more or less amphithect or bilateral. But no 

 larva of Porpita has the vertical sail or crest shown in his figs. 3-5, &c. ; these belong 

 to Velella. 



The medusiform gonophores, which bud in clusters from the gonostyles of the 

 Velellidae, and afterwards become detached (Discomitra, Chrysomitra), are described by 

 many authors (mainly by Kolliker, 4, Vogt, 5, Gegenbaur, 7, Leuckart, 8, Huxley, 9, Alex- 

 ander Agassiz, 57, and others (compare on the metagenesis of the Velellidae, p. 39 above). 



Phylogeny. — The Velellidae have probably descended from the older forms of 

 Porpitidae (or directly from the Discalidae) ; their peculiar amphithect or bilateral develop- 

 ment is due to the production of the vertical sail as a longitudinal fold of the 

 exumbrella. 



Synopsis of the Genera of Velellidae. 



1. Umbrella elliptical, with integral margin. Pneumatocyst without crest. Corona of 



tentacles simple, . . . . . . . . .7. Rataria. 



2. Umbrella elliptical or quadrangular, with integral margin. Pneumatocyst with a crest. 



Corona of tentacles simple, ....... 8. Velella. 



3. Umbrella quadrangular, with a lobate margin. Pneumatocyst with a crest. Corona of 



tentacles double or multiple, ....... 9. Armenista. 



Genus 7. Rataria, 2 Eschscholtz, 1829. 

 Rataria, Esch., System der Acalephen, p. 166. 



Definition. — Velellidae with an elliptical umbrella and a vertical membranous sail 

 placed in the major axis of the ellipse. Margin of the umbrella not lobate. Pneumato- 

 cyst flatly campanulate or more discoidal, elliptical, without vertical chitinous crest. 

 Submarginal corona of tentacles simple. 



The genus Rataria is the simplest and most primitive form of the Velellidae, and 



1 1, p. 166. 2 Rataria= A float. 



