The tribe Botryllina (les Botrylliens, Milne Edwards) comprises those compound Asci dians that are united in systems around common excretory cavities or eloacce, and whose bodies are not divided into a distinct thorax and abdomen, the viscera being pushed forward on the side of the branchial cavity, and form ing with the thorax an ovoid mass.
Genus Botryllus, Gaertner ; synonyms, Al cyonium, Am., Polyeythes, Lamarck.— Com mon body gelatinous or cartilaginous, sessile and incrusting ; systems numerous, pro minent, round or star•shaped, with central cavities; individuals, six to twenty in each system, lying horizontally with the vent far from the branchial orifice; branchial orifice simple (fig. 771.).
Genus Botrylloides, Milne-Edwards.— This genus resembles the foregoing in most respects, except that the stars formed by the systems of animals are irregular and ramifying ; the cloacw being prolonged into the common mass as irregular internal channels, on each side of which the individuals are placed in linear series, instead of having a simple star like arrangement around the cloaca', as in Botryllus. The animals of Botrylloides, more over, have a nearly vertical position, and their orifices are closely approximate (fig. 783.).
We should perhaps also refer to the group f the Botryllidre, an obscure form, first no ticed by Molina*, and subsequently named Pyura by Blainville, and considered by the latter to form a link between the simple and compound Ascidians. M. Blainville gives the following characteristics.
Genus Pyura, Blainville.—A pyriform body, with two small short tubes, occupying a cell in the external envelope, and forming, by its union with 10 to 12 individuals, a kind of poly morphous mass somewhat resembling honey comb, apparently without any external orifice.
Family PYROSOMID/E. Synonyms : Lucia compas6es, Savigny ; Tuniciers rounis, pars, Lamarck ; " les agriges," pars, Cuvier ; Sal piens agrhges, Blainville ; Lucidce, MacLeay; Pyrosonziens, Milne-Edwards and Van Bene den. The Pyrosoinians are represented by three
or four species of the single genus Pyrosonza.
They inhabit the Mediterranean and the warmer parts of the ocean ; in the former at times their abundance is a source of dread to the fishermen, sometimes even completely clogging their nets ; and in certain oceanic regions they are met with in almost incredible profusion. Their delicate and transparent forms, their elegant tints, and their unrivalled phosphorescence render them the most beau tiful of Molluscs, and objects of admiration to the naturalist and the voyager. Mr. Bennett relates that, during a voyage to India, the ship, proceeding at a rapid rate, continued during an entire night to pass through distinct but extensive fields of these Molluscs, float ing, and glowing as they floated, on all sides of her course. Enveloped in a flame of bright phosphorescent light, and gleaming with a greenish lustre, the'Pyrosomes, seen at night, in vast shoals upwards of a mile in breadth, and stretching out till lost in the distance, present a spectacle the glory of which may be easily imagined. The vessel, as it cleaves the gleaming mass, throws up strong flashes of light, as if ploughing through liquid fire, which illuminates the hull, the sails, and the ropes with a strange unearthly radiance.
Genus Pyrosonza, Peron. — Common body semi-cartilaginous, floating, cylindrical, 2 to 14 inches long, to 3 inches in circumference ; bearing externally numerous pointed pro cesses, hollow and mainmillated within, and open at one of its extremities only. Animals associated in a verticillate arrangement, having two orifices, one at each extremity ; elongated, fusiform, tapering at the outer, and obtuse at the inner extremity ; united at the circum ference of the middle portion, by the fusion of the tests to one another into rings, more or less regular, and varying in number accord ing to species, so that the whole forms the long cylinder above described.