BRANCHIO'STOMA, the name given by Costa to the most anoma lous of all living fishes, and indeed of all the Vertebrate.
This extraordinary animal was first discovered on the coasts of Britain, a single specimen having been sent to Pallas from the coast of Cornwall during the latter part of the last century. The great naturalist of Russia described and figured it in his Spicilegia Zoolo gica' under the name of Linter lanceolatus, believing it to be a mollusk, though remarking in his description of it on the resemblance of some of its characters to those of a fish. It seems to have been lost sight of for more than half a century, and with the exception of a brief reference in Stewart's 'Elements of Natural History' we find no notice of it in any synopsis of animals. In 1834 it was re-discovered by Costa on the Neapolitan shores, who described it in the 'Annuario Zoologico ' under the name of Branchiostoma lubricum ; and some years after in his 'Fauna of the Kingdom of Naples' gave a fuller account of it. Costa first perceived that it was a fish and not an invertebrate animal, and remarked its affinity to the Cyclostomatous fishes. In 1836 Mr. Yarrell gave an account of it in his 'History of British Fishes' under the name of the lanceolatus). He had not then met with Costa's account of it. He figured and described it from a specimen found by Mr. Couch at Polperro in Corn wall, the first taken in that locality since its original discovery there. Mr. Yarrell gave the first correct notice of the chords. dorsalis and vertebral column. About the same time, singularly enough, consider ing how long it had escaped notice since the days of Pallas, it was taken by several naturalists on the coasts of Sweden. Lundevall and Lovcn found it in Bohuslan in 1834, but did not give an account of it till 1841. Retzius had it from the same locality, and published a notice of it in the 'Berlin Proceedings for November,' 1839, in which also is a communication on the same subject by Professor J. Muller. Bethke gave an account of its structure in 1841. In the same year Mr. J. Goodsir published an elaborate memoir on its anatomy in the 'Transactions of"the Royal Society of Edinburgh' for 1841, being the result of his examination of two examples taken in the Irish Sea by Professor E. Forbes in 1837. In 1842 a most valuable memoir on this animal was read before the Royal Society of Berlin by Professor J. Milner, and this paper beautifully illustrated appeared in the volume of 'Transactions' of that society published in 1844.
Besides the Instances of its capture above mentioned it has been since taken by Mr. MacAndrew on the west coast of Scotland, and by Professor Edward Forbes in the lEgean Sea, and by those gentle men on the south coast of England in 1846.
The great interest which attaches to this fish depends on the strangeness of its anatomical characters, the unexampled degradation of its organisation among the Vertebrate, and the link which it forms between the highest of animals and some of the lowest. A verte brated animal without a brain, a fish with the respiratory system of a mollusk, and the circulatory almost of an Annelide, presents a combination of characters which must excite the wonder and interest equally of the physiologist and the systematic naturalist. Scarcely any animal yet discovered is so likely to change received views of classification and relative order of characters as the Lancelet. As yet however it has attracted but little attention among zoologists, though the physiologists and anatomists have fully perceived its value. For these reasons we shall give a full account of what is now known respecting its external character, structure, and habits.
The usual size of the Lancelet is about 2 inches in length ; the height to the length being as I : 10, and tho breadth to the length as : 10. It is of a lanceolate form tapering to each extremity, and riband-like. Anteriorly it terminates in a head scarcely distinguish able from the body, apparently pointed, but when examined closely seems to end in a rounded and somewhat spathulato rostrum, beneath which is the mouth, a longitudinal opening, fringed on cach side by a row of long filaments which can close in and clasp alternately, so as to protect the oral opening. Along the back runs a continuous fin, which dilates near the sharp posterior extremity on each side so as to form a sort. of Caudal fin. Near the tail opens the vent, in front of which is a median fin continued to another opening situated a little behind the centre of the body (porua abdominalis), and serving as an outlet for the genital products. Continued from this forwards nearly to the mouth are two strong lateral folds, mistaken by Pallas for the margins of a ventral disk, and hence leading him to consider the animal a Gasteropodous Mollusk. The entire animal is translucent and of a silvery whiteness, its aides being marked by the indications of the lateral ichthyic muscles, which give it the aspect of a small sand-eel.