LOCOMOTION OF TUNICATA.- Generally speaking, the muscular tissue of the Aseidiadw, Botryllidee, and Pelonaiadw is not subservient to the purposes of locomotion, and merely effects the sudden removal of water and noxious objects from the branchial sac. The entrance and exit of water through the ex ternal orifices, constituting the chief sign of vitality in these apathetic creatures, are ge nerally caused by the ciliary currents.
But the water may be driven out by an ejaculatory effbrt of the constrictor muscles of the mantle ; which action would be followed by a more or less rapid expansion or dilata tion of the mantle and test, effected by the elasticity of the latter. In such species as occasionally occur unattached, or even float ing about freely, such an ejaculatory action would effect a transitory retrograde move ment; but we have no evidence that any of the Ascidiadce or Pelonaiedce make use of this propulsive agent in the manner of the Pyro somata and the Salpw. Nearly all the Asci diadce and Botryl/idw are sessile, but some few, mounted on flexible pedicles as Boltenia and Sigillina, may be said to enjoy a limited freedom of movement, necessary perhaps to their well being ; and the same may be said of such species as are attached to the floating branches of flexible algm and corallines.
According to Mr. F. D. Bennett, except in the action of the sphincter-like membrane, surrounding the open extremity of the com pound cylinder, there was but very slight evidence of motive power in the specimens of Pyrosoma examined by him. NEV. Peron and Lesueur, however, describe a slight retrograde motion observable in this animal. The method in which this is brought about is not very clear ; it appears to be caused either by the synchronous contraction of the individual animals, causing a diminution in the general calibre of the cylinder, and thus effecting a faint ejaculation of water from the cavity of the latter ; or by the posterior current of water from the anal orifice of each animal being synchron ously ejected into the cavity of the cylinder, and thus giving a motive power to the whole.
We have a more decided locomotion ex hibited in the Salpa, which is treated of by Mr. Bishop* as a " syringograde" animal, in the same category as "the Holothuria, and the larva; of those insects whose progression is effected by the alternate reception and expul sion of water to and from their respiratory organs by an action similar to that of the syringe." In Salpa the dilatation of the test and its membranous lining causes the water necessary for respiration and nutrition to enter through the bilabiate posterior orifice, which has a valve preventing the return of the water by the same aperture t; a trans verse contraction of the body then expels the water through the anterior orifice, and the result is, that the animal is forced backwards, being carried in an inverse direction to that of the ejected water. t This retrograde motion has caused the posterior orifice to be regarded by some naturalists as the true oral orifice. The alternate action, of dilatation and contraction, have been observed to take place about fifteen times in a minute ; and have sometimes been termed the systole and the diastole. The contraction is effected rapidly, but the relax ation, or rather the dilatation of the sub elastic test, takes place but slowly. These movements are synchronous throughout a chain or group of Salpec.
After all, this is but an imperfect amount of locomotion, and it is extremely probable that both the single Salpce and the Salpman wreaths and chains, the latter often several ) ards in length, are, like the feebly moving Pyrosoma, the sport of wind and wave, wafting them hither and thither, either to bask calmly in the sunshine, or to be broken on the rocky shore.