ACALEPHAE In the third division of Acalephre, denominated by Cuvier, Acalephes Hydrostatiques,' the body is supported in the water by a very peculiar organ, or set of organs, provided for the pur pose. This consists of one or more bladders, capable of being filled with air at the will of the'animal, which are appended to the body in various positions, so as to form floats of sufficient buoyancy to sustain the creature upon the surface of the sea, when in a state of distension ; but, when partially empty, allow ing it to sink, and thus escape the approach of danger." " The most conspicuous, if not the most typical, member of the Physo grade order of Acalephee is the Physalia, (known to sailors by the name of the Portuguese man-of-war,) in which all that part answering to the disc in the Pulmograde' order, is expanded into a bag, the major part of which is occupied by an air-bladder, whilst the digestive cavity is subdivided amongst a series of ap pendages attached to one part of the under surface of the bag.
This part consists of an outer thin and dense membrane, of an inner thicker membrane beset with long cilia, and of an air bladder, which at one point is attached to the above membranes, where there is a small constricted aperture, at least in the outer membrane. This membrane is developed into a kind of crest along its upper part." "The air-bladder is endowed with a con siderable power of contraction, and when carefully examined, two orifices are observable, one at each extremity, through which, upon pressure, the contained air readily escapes, a provision for enabling the creature to regulate its specific gravity at pleasure, and, when alarmed, at once to lessen its buoyancy by diminish ing the capacity of its swimming-bladder, and to sink into the waves." " The opinion that these animals are able to expel the air from the air-bladder at will, was rendered- doubtful, as a general rule, by Olfers, who could find no opening in the large bladder of Physalia. [Subsequent observations, however, have determined that Physalia is the only one of the whose bladder does really communicate with the external air. But, though there be no such communication in the rest, Leuckart states that in many of them (and he believes it to be true of all,) the air may be readily caused to pass from the cavity of the bladder into that of the common stem, by the expansion of the upper extremity of which the air-bladder is in all cases sur rounded." "Quatrefages has described the action of the sphincter
muscle, and the connexion of both bladders with the aperture ; he also caused the air contained in the interior bladder to be analyzed, and found that it contained less of oxygen than atmo spheric air, by about three per cent.; the animal appeared to be able to expel the air voluntarily at intervals, and to distend the bladder again after a short time ; it would therefore seem to be a respiratory organ for the colony. The air-bladder is surround ed on all sides by the external bladder or envelope, which is, in fact, the expanded stem of the colony ; with the under surface of this the various appendages are connected, and into its cavity the cavities of them all open more or less directly. The bladder in Physalia did not appear to Quatrefages to be merely a passive organ, for besides the power of emptying and distending it, the animal seemed to be able to direct the fluid contained in the cavity of the appendages into this or that bundle of them at will, and so to alter the position of the centre of gravity of the blad der, and by thus bringing different regions of it to the surface, to steer its course.] The appendages are of three kinds: Urticating, digestive, and (probably) generative. The urticating tentacles are the longest ; they are hollow, and are provided with muscular fibres, of which the most conspicuous are longitudinal, and serve to retract them. They contain many corpuscles of a reniform shape, and are richly provided with thread-cells, whose filaments are of the spiral kind. The gastric appendages are shorter and wider, and are provided with stomata, which are applied to the prey seized and benumbed by the tentacles. If the prey be small, it is sucked bodily into the gastric sac ; if large, the sac becomes distended with its juices and dissolved parts, the gastric secretion being a very rapid and powerful solvent. The mouth of each sac is wide, with a broad everted lip, armed with a series of ' nettle cells.' The whole gastric appendage is highly contractile, and in constant motion in the living animal. The appendages of the third class are cyathiform."