being cartilaginous instead of bony; in the present Family the latter distinctive mark gradually disappears, the spine in the highest forms being “traversed by a single tendinous cord, filled internally with a mucilaginous fluid, without contractions and enlargements, so that the vertebrae are reduced to cartilaginous rings not easily distinguishable from each other, and, indeed, not cartilaginous through their whole circle,” (Cuvier);–while in the lowest forms (Amphioxus), it is reduced to a simple cartilaginous column or thread, flexible, transparent, and scarcely to be distinguished from the horny pen enveloped in the flesh of some of the Mollusca. Hence it has been disputed whether these minute creatures have a right to a place among Vertebrata; though the preponderance of opinion, founded on dissection and comparison of various organs, is in favour of such a position being assigned to them.
What we have further to say must be considered as applying principally to the more developed members of the Family. They have no pectorals nor ventrals; but foldings of the skin above and below the hinder parts of the body serve the purpose of dorsal, caudal, and anal; destitute, however, of supporting rays. The form is long, slender, and cylindrical, much resembling that of a worm; the mouth consists of a circular fleshy lip, with a cartilaginous ring supporting it. The gills are not comb-shaped fringes, but form sacs or pouches, by the union of two opposite ones along their edges.
These humbly organized members of the great Vertebrate division of animated beings are but
