580 
MR. HUXLEY ON THE ANATOMY OF SALPA AND PYROSOMA. 
into a common mass, but an individual which is developed into a greater or less 
number of zooid forms, which in the present case remain united. 
Thus the stem and branches of the polypidom in Sertularia are “ organs,” the 
ovarian vesicles are “organs,” the polypes are “ zobids ;” the sura of the organs and 
zooids constitutes the individual. 
If the separate polypes be individuals, what is the polypidom which exists before 
them, and therefore cannot be derived from them ? 
It seems startling to assert that a Salpa of the form A with some fifty or sixty 
of the form B which proceed from it, constitutes but one zoological individual ; still 
more to aver this of some millions of Aphides all proceeding indirectly from one 
ovum ; but these difficulties have reference merely to our ordinary notion of indivi- 
duality, and involve us in no self-contradictions and inconsistencies such as seem 
inherent in any other view of the case. 
Section II . — The Anatomy of Pyrosoraa. 
41. This genus, first established and very imperfectly described by Peron*, 
received elaborate investigation from Lesueur-I- and from Savigny:|;, who very care- 
fully described every part of its organization with the exception of the generative 
organs, and one or two other points of minor importance. 
Subsequently M. Milne-Edwards§ showed that the nature of the circulation was 
the same in it as in the other Ascidians. 
Of the three species distinguished by Lesueur the present appears most closely to 
resemble the P. atlanticum. 
42. The only specimen of this remarkable animal which the writer has had an 
opportunity of examining in the fresh state, was procured on the night of the 15th 
of June 1850, in about 45°‘85 S. lat. and 110°'30 W. long. The sky was clear but 
moonless, and the sea calm ; and a more beautiful sight can hardly be imagined than 
that presented from the decks of the ship as she drifted, hour after hour, through 
this shoal of miniature pillars of fire gleaming out of the dark sea, with an ever- 
waning, ever-brightening, soft bluish light, as far as the eye could reach on every 
side. The Pyrosomata floated deep, and it was with difficulty that some were pro- 
cured for examination and placed in abucketfull of sea-water. The phosphorescence 
was intermittent, periods of darkness alternating with periods of brilliancy. The 
light commenced in one spot, apparently on the body of one of the “ zooids,” and 
gradually spread from this as a centre in all directions ; then the whole was lighted 
up; it remained brilliant fora few seconds, and then gradually faded and died away, 
until the whole mass was dark again. Friction at any point induces the light at 
that point, and from thence the phosphorescence spreads over the whole, while the 
* Aniiales du Museum, 1804. 
I Mem. sur les Animaux sans Vertebres, 
t Journal de Physique, 1815. 
§ Comptes Rendus, 1840. 
