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



numerous and closely placed, leaving elongated slits between, whicli are 

 transversely directed ; and longitudinal vessels, which are not quite so 

 numerous, and cross the transverse vessels so as to form small quadrangular 

 meshes. 

 Tentacles simple, one ventrally placed, larger than the rest. 

 Dorsal Lamina represented by a series of eight or more tapering languets. 

 Dorsal Tiiheirlc, with a simple aperture. 



Alimentary Canal short and simple, placed posteriorly to the branchial sac. 

 Reproductive Organs hermaphrodite, placed posteriorly to the branchial sac, in 



diverticula of the peribrauchial cavity. 

 Gemmation from a ventrally-placed stolon formed at the posterior end of the 

 endostyle. 

 This genus was founded by Peron in 1804.^ His specimens were obtained in the 

 equatorial Atlantic between Mauritius and Europe during a voyage round the world 

 along with Lesueur, and were very imperfectly described and figured in his memoir, 

 and also in the account of the voyage. Tke genus is characterized very briefly as 

 follows : — " Pyrosoma. Corpus lihcrum, suhconicum, extremitate ampliore apertum, 

 vacuiim, aperturce margine intus tuherculis cincto." The further description is 

 worthless. 



The specific description : — " Pykosoma atlanticum. ^quatorio-atlanticum, 

 gregarie-pelagivagtmi vividissime p)li,osphoresGens, colorihus eximiis tunc effidgens in 

 aquis viginti duohus reaumwianis calidiorihus occurens, 10-12-14-16 ccntimetros 

 CBquans," contains no character which is of any use in determining the species ; and 

 the animal was evidently regarded by its first investigator as a zoophyte, consisting of 

 a single polype of which the terminal aperture was the mouth, while the Ascidiozooids 

 were interpreted as simple elongated glands having a phosphorescent function. The 

 plate, drawn by Lesueur, is very little better than the description. It represents the 

 external appearance and a longitudinal section of tlie colony. 



Lamarck^ shortl}' afterwards placed Pyrosoma in the Eadiata beside Beroe, but 

 added nothing to the knowledge of its nature. 



Peron and Lesueur again met with Pyrosoma in a voyage to Nice, and a new 

 species then obtained was described by Lesueur^ in 1813 under the name oi Pyrosoma 

 elegans. This differed from Pyrosoma atlanticum in having the tubercles on the 

 surface arranged regularly in verticils. A third species which was found at Nice 

 was discussed by Lesueur * in a more important paper, Memoire sur I'organisation des 



' Memoire sur le nouveau genre Pyrosoma, Ann. Miis. Ilist. Nat., torn. iv. p. 437 ; also, Peron and Lesueur, 

 Voyage aux Terres austr., torn. i. p. 488, pi. 30, fig. 1. 



2 Philosopliie Zoologique, torn. i. p. 294, 1809. 



3 Nuuv. Bull. Soc. Philomath., torn. iii. p. 283. 

 * Joitrnnl dc Physique, torn. Ixxx. p. 4131 



