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XVI. On the Anatomy and Development of Pyrosoma. By Thomas H. Huxley, Esq., 
F.R.S.y F.JL.S., Sec. G.S., Professor of Natural History in the Government School 
of Mines. 
Read December 1st, 1859. 
1. History of the Genus Pyrosoma. 
THE genus Pyrosoma was first established in 1801 by Peron, in a memoir* published 
in the fourth volume of the * Annales du Museum/ and accompanied by a plate repre- 
senting the exterior and a longitudinal section of the animal. Peron thus defines the 
genus and the species which he observed : 
" Pyrosoma. 
" Corpus liberum, subconicum, extremitate ampliore apertum vacuum, aperturae margine intus tuberculis 
cincto. 
"Pyrosoma atlanticum. iEquatorio-atlanticum, gregarie pelagivagum, viridissime 
phosphorescens, coloribus eximiis tunc effulgens, in aquis viginti duobus reaumu- 
rianis calidioribus occurrens, 10-12, 14-16 centrimetros aequans. 
5> 
M. Peron's conceptions of the exigencies of a zoological diagnosis were evidently of a 
singular kind, and his memoir contains not a single observation calculated to throw 
light upon the true nature of one of the most remarkable animals that has ever been 
discovered. With respect to the striking property which gave rise to the name conferred 
on the genus, Peron asserts that the Pyrosomata exhibited movements of alternate con- 
traction and dilatation at regular intervals ; and that each contraction was accompanied 
by the development of a luminosity, which, when at its brightest, was red, but, in dying 
away, passed through shades of orange, green, and blue. The light was developed upon 
irritation, and entirely ceased with the animal's death. The only indication of locomotive 
power was the regular contraction just described, whose necessary effect was a slight 
retrogressive movement, in consequence of the reaction of the water forced out of the 
open end of the Pyrosoma. 
In 1815, Lesueur, having previously, as he states, described and figured a new species 
(P. elegans) in the < Nouveau Bulletin de la Societe Philomatique ' for 1813, added a num- 
ber of important details to Peron's account in his « Memoire sur l'organisation des 
* " Memoire sur le nouveau genre Pyrosoma," par M. Peron. Annalea du Museum, torn. ir. p. 437, 1804. 
Forskal's Deacriptiones Animalium (17/5) contains the following passage : 
" 29. Medusa Beroe. Tres ejus varietates vidi vel species. * * 
"29c. rufescens: ovato-oblonga ; sajpe 5 poll, longa; intus prorsua vacua. Gallice, Coneombre de la mer. In 
mari Mediterraneo frequens." 
Was Medusa Beroe rufescens a Pyrosoma ? 
VOL. XXIII. D 
