This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
In the third division of Acalephae, denominated by Cuvier "Acalephes Hydrostatiques," the body is supported in the water by a very peculiar organ, or set of organs, provided for the purpose. 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, allowing it to sink and thus escape the approach of danger. In Physalia (fig. 61), known to sailors by the name of the "Portuguese man-of-war," the swimming-bladder is single, and of great proportionate size, so that when full of air it is exceedingly buoyant, and floats conspicuously upon the waves. The top of this bladder bears a crest, c, of a beautiful purple colour, that, presenting a broad surface to the wind, acts as a sail, by the assistance of which the creature scuds along with some rapidity. The air-bladder is endowed with a considerable power of contraction, and when carefully examined, two orifices are observable, one at each extremity, a, b, 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 diminishing the capacity of its swimming-bladder, and to sink into the waves. The nature of the air with which the little voyager distends its float has not been accurately determined; but it is undoubtedly a secretion furnished at pleasure when at a considerable distance from the surface, although the mode of its production is still unknown.
The Cirri-grade Acalephae form a very remarkable family, peculiarly distinguished by the possession of an internal solid support, or skeleton, secreted in the substance of their soft and delicate bodies. In Porpita (fig. 62), this consists of a flat plate of semicar-tilaginous texture (fig. 62, 2), evidently deposited in thin secondary laminae, which gradually increase in size as the animal advances in growth, the inferior being the largest and last formed. When examined after its removal from the body, this fragile skeleton is seen to be extremely porous or cellular; and the pores being filled with air, it is specifically lighter than water - a circumstance that may contribute to the buoyancy of the creature even when alive.
(328). The lower surface of Porpita is furnished with numerous appendages called cirri, whereof some appear to be organs of prehension, but perform also the office of oars, which, in this species, are the principal agents in progression; yet in other Cirrigrada, as Velella and Bataria, besides the horizontal lamella that forms the whole skeleton of Porpita, there is a second subcartilaginons plate, rising at right angles from its upper surface, and supporting a delicate membranous expansion, that rises above the water and exposes a considerable surface to the wind, so as to form a very excellent sail. To perfect so beautiful a contrivance, in Rataria the crest is found to contain fibrous bands, apparently of a muscular nature, by the contractions of which the sail can be depressed or elevated at pleasure.
Fig. 61. Physalia. a b, vesicular float; c, crest; d, orifice; e, nucleus; f f, inferior append.
Fig. 62. Porpita.
The last family of the Acalephae. derives its name from the singular appearance of the creatures composing it; each animal, in fact, seems to consist of two portions (a, b, fig. 63,1, 2) so slightly joined together, that it is by no means easy to understand the nature of the connexion between them.
Fig. 63. Diphyes Bory.
(330). The body of these strangely-organized beings is composed of two polygonal, subcartilaginous, transparent pieces placed one behind the other, the posterior division being implanted more or less deeply into the anterior. These two divisions are invariably more or less dissimilar from each other; nevertheless they offer this circumstance in common, that they are excavated internally by a deep cavity, which opens externally with a wide orifice of regular shape, although differing in form in each division. To these details of their general appearance must be added the existence of a long filiform appendage, which issues from the upper cavity of the anterior cartilaginous portion, and which was regarded by Cuvier as the ovary.
(331). On more minute examination, there is recognizable in the anterior division a visceral mass called the nucleus, which is made up of a proboscidiform oesophagus, terminated by a sucker-like mouth, and continuous with a stomachal cavity, whereunto are appended hepatic follicles of a greenish colour, and sometimes a little vesicle filled with air.
(332). Besides the above structures there may be remarked, towards the lower part of the body, another glandular-looking mass, probably the ovary, which is connected with the long (ovigerous?) filament above alluded to. The nucleus is contained in a proper cavity, generally distinct from the large excavation that forms the locomotive apparatus, and is connected by filaments, apparently of a vascular character, to the soft parts within the body. It has been already remarked that this latter division is excavated by a large cavity that extends nearly throughout its entire length; from the bottom of this cavity arises a prolongation, probably of a vascular character, which embraces the root of the (ovigerous?) filament, and is apparently connected with the nucleus, from which, however, it may be detached by the slightest effort.