Two arching vessels, one on either side, pass from near the nerve-ganglion to the com pressed oval bodies on the anterior lateral points of the branchim, and a similar pair of vessels unite these latter bodies with the extremity of the loop formed by the vessel that lies between the free ventral margins of the branchim. These four arched vessels form therefore a circlet around the anterior ex tremity of the branchial sac (fig. 786. B, h).
The heart is placed at the posterior part of the body, at the side of, and below, the visceral mass. Its character is perfectly analogous to that of the heart of other Ascidians. It con tracts with the usual peristaltic movement, and changes periodically the direction of this vermicular movement, the vessels alternately playing the part of artery and vein.
With regard to the generation of the Pyroso midw, very little has yet been observed. Sa vigny describes as "oviduct" and "siphon canal" the vessels that occupy the usual position of the dorsal sinus, running along the dorsal or superior surface of the branchial sac (fig. 786. g, g). They have, however, no apparent connection either with the pre sumed oviferous bodies situated at the an terior points of the branchim, or with the cavity posterior to the abdominal viscera occupied by the ova in Savigny's specimens (fig. 786. A, b). Lesueur discovered and described cer tain globular, transparent bodies in Pyrosonia, situated near the liver ( fig. 786. c, k), which he regarded as ova ; these were noticed also by Savigiiy. Each enclosed four minute Py rosomes, symmetrically disposed, and readily recognised by the form and arrangement of their double branchhe.
Whether the ova of Pyrosoma be always composed of four, or a greater or less number of young individuals, or whether they ge nerally contain but a single one, it is still highly probable that genzmiparous reproduc tion obtains with these animals ; the fissi parous mode, however, imagined by Peron as possibly occurring in the adult Pyrosome, is totally unsupported by evidence. The several stages of development in which the individual animals are found to exist in the common mass of the test, as noticed by Savigny, point to this conclusion ; and possibly the delicate filaments regarded as muscular fibres by Sa vigny, first pointed out by the acute observer of these creatures, Lesueur, as traversing the test in a line with the abdominal cavities of the adult animals (fig. 786. is, c), may be the proliferous stolons as yet untraced throughout their course.
Phosphorescence of Pyrosoma. — M. Peron, who first discovered and established the genus, has given a lively description of the circum stances under which the P. atlanticum were first met with by him, in his Memoire sur le Pyrosome.* "We had," says he, "for a long time been detained by calms in the equatorial regions, between 19° and 20° long. west of Paris, and 3° and 4° north lat., the temperature of the sea being at the surface 22° Reaumur ; and we could make no progress except by the aid of the short-lived storms peculiar to these climates. In the evening of the 13th Frimaire we experienced one of the strongest of these squalls ; the sky was on all sides loaded with heavy clouds ; all around the obscurity was profound ; the wind blew violently; and the ship cut her way with rapidity. Suddenly we discovered, at some distance, a great phos phorescent band stretched across the waves, and occupying an immense tract in advance of the ship. Heightened by the surrounding circumstances, the effect of this spectacle was romantic, imposing, sublime, rivetting the at tention of all on board. Soon we reached the illuminated tract, and perceived that the prodigious brightness was certainly and only attributable to the presence of an innumerable multitude of largish animals floating with the waves. From their swimming at different depths they took apparently different forms ; those at the greatest depth were very inde finite, presenting much the appearance of great masses of fire, or rather of enormous red hot cannon-balls ; whilst those more distinctly seen near the surface perfectly resembled in candescent cylinders of iron.
" Taken from the water, these animals per fectly resembled each other in form, colour, substance, and the property of phosphores cence, differing only in their sizes, which varied from 3 to 7 inches. The large, longish tubercles with which the exterior of the Pyro sorties was bristled were of a firmer substance and more transparent than the rest of the body, and were brilliant and polished like diamonds. These were the principal seat of phosphorescence. Between these large tu bercles, smaller ones, shorter and more ob tuse, could be distinguished ; these also were phosphorescent. Lastly, in the interior of the substance of the animal, could be seen, by the aid of the transparency, a number of little, elongated, narrow bodies (viscera), about a millimetre in length, which also participated in a high degree in the possession of the phosphoric light.