ANTERO-POSTERIOR SYMMETRY.— By re ference to the diagram at page 824. Vol. Ill. Fig. 433., representing the abstract notion or type of a vertebral segment, it will be seen that the upper or posterior half is a reverse of the lower or anterior. Referring to what really exists in nature, we find, in the caudal vertebrm of fish, that their dorsal and ven tral halves are counterparts tolerably exact, yet that exactness is not nearly what exists between the lateral halves. If an antero posterior as well as a lateral symmetry be admitted, then we have four repetitions ar ranged around a centre.. At all events there is here, in the Vertebrata, some amount of evidence of radiation or divergence from a central axis comparable in some degree to what we see in the Radiata. The anterior and posterior parts of vertebral segments, as [found in nature, are usually extremely dissimilar. The rays of the dorsal and ab-; dominal (anal) azygos fins of fishes are an tero-posterior repetitions, but the fan-like tail fin, which in most fishes seems to be remarkably symmetrical antero- posteriorly, half of it apparently belonging to the dorsal and the other half to the abdominal moiety of the fish, usually belongs, as I have ob served in the typical fishes, cyprinoid, &c., in reality, entirely to the abdominal or under moiety. The embryo of these fishes at first has a tail like an eel, into which the spinal column is continued nearly to Its tip. A little way from the extremity of
this, on the abdominal aspect, a group of fin rays are soon observed to sprout out meanwhile that the end of the vertebral co lumn shrinks and turns up. The group of fin rays grows, and the vertebral column shrinks, so that in time the former is brought to form the fan-like extremity of the animal ; but even then the atrophied end of the verte bral column may be detected occupying the upper edge of the fan. Even in the adult fish some trace of this original relation of the tail fin can be detected. If the tail of an adult homocercal fish be macerated or boiled, and all the pieces which are not an chylosed to it be removed, what remains will not be symmetrical, but will terminate by turning or cocking up. Even in the Pleuro nectidre, whose tails seem to be remarkably symmetrical, and where the spinal column seems to terminate in a flat triangular piece, it will be found that the lower half of this piece can be easily removed, whilst the upper half forms one piece with the body of the last vertebra, with which, in fact, it forms a coccyx composed of numerous degenerated and consolidated vertebrae.