This order includes those Siphonophora whose hydrosoma is free and oceanic, and is propelled by " nectocalyces" attached to its proximal end. The hydrosoma consists of several polypites, united by an unbranched coeno-sarc, which is highly flexible and contractile, and never develops a hard cuticular layer. The proximal end of the hydrosoma is modified into a peculiar cavity called the " somalocyst." The reproductive organs are in the form of medusiform gonophores produced by budding from the peduncles of the polypites.
In all the Calycophoridae the coenosarc is filiform, cylindrical, unbranched, and highly contractile, this last property being due to the presence of abundant muscular fibres. "The proximal end of the coenosarc dilates a little, and becomes ciliated internally, forming a small chamber "which communicates with the nectocalycine canals. "At its upper end this chamber is a little constricted, and so passes, by a more or less narrowed channel, into a variously-shaped sac, whose walls are directly continuous with its own, and which will henceforward be termed the somatocyst (fig. 54, 3 b). The endoderm of this sac is ciliated, and it is generally so immensely vacuolated as almost to obliterate the internal cavity, and give the organ the appearance of a cellular mass" (Huxley). The polypites in the Calyco-phoridae often show a well-marked division into three portions, termed respectively the proximal, median, and distal divisions.
Fig. 54. - Morphology of the Oceanic Hydrozoa. 1. Diagram of the proximal extremity of a Physophorid; a Pneumatocyst. 2. Vogtia pentacantha, one of the Calyco-phoridae: n Nectocalyces; p Polypites; t Tentacles. 3. Diagram of a Calyco-phorid: a a' Proximal and distal nectocalyces; b Somatocyst; c Coenosarc; d Hydrophyllium or bract; e Medusiform gonophore; f Polypite. The dark lines in figs. 1 and 3 indicate the endoderm, the light line with the clear space indicates the ectoderm. (After Huxley.)
Of these the "proximal" division is somewhat contracted, and forms a species of peduncle, which often carries appendages. The "median" portion is the widest, and may be termed the " gastric division," as in it the process of digestion is carried on. It is usually separated from the proximal division by a valvular inflection of the endoderm, which is known as the "pyloric valve." The polypites have only one tentacle "developed near their basal or proximal ends, and provided with lateral branches ending in saccular cavities," and furnished with numerous thread-cells. The proximal ends of the polypites usually bear certain overlapping plates of a protective nature, which are termed "hydrophyllia" or "bracts." They are composed of processes of both ectoderm and endoderm (fig. 54, 3 d), and they always contain a diverticulum from the somatic cavity, which is called a "phyllocyst." The Calyco-phoridae always possess swimming-bells, or "nectocalyces," by the contractions of which the hydrosoma is propelled through the water (fig. 54, 2). The nectocalyx in structure is very similar to the "gonocalyx" of a medusiform gonophore, as already described; but the former is devoid of the gastric or genital sac - the "manubrium" - possessed by the latter. Each nectocalyx consists of a bell-shaped cup, attached by its base to the hydrosoma, and provided with a muscular lining in the interior of its cavity, or "nectosac." There is also always a "velum" or "veil," in the form of a membrane attached to the mouth of the nectosac round its entire margin, and leaving a central aperture. The peduncle by which the nectocalyx is attached to the hydrosoma conveys a canal from the somatic cavity which dilates into a ciliated chamber, and gives off at least four radiating canals, which proceed to the circumference of the bell, where they are united by a circular vessel; the entire system constituting what is known as the system of the "nectocalycine canals." In the typical Calycophoridae two nectocalyces only are present, but in some genera there are more. In Praya the two nectocalyces are so apposed to one another that a sort of canal is formed by the union of two grooves, one of which exists on the side of each nectocalyx. This chamber, which is present in a more or less complete form in all the genera, is termed the "hydrcecium," and the coenosarc can be retracted within it for protection.
The reproductive bodies in the Calycophoridae are in the form of medusi-form gonophores, which are budded from the peduncles of the polypites, becoming, in many instances, detached to lead an independent existence. In some Calycophoridae, as in Abyla, "each segment of the coenosarc, provided with a polypite, its tentacle, reproductive organ, and hydrophyllium, as it acquires a certain size, becomes detached, and leads an independent life - the calyx of its reproductive organ serving it as a propulsive apparatus. In this condition it may acquire two or three times the dimensions it had when attached, and some of its parts may become wonderfully altered in form" (Huxley). To these detached reproductive portions of adult Calycophoridae the term "Diphyozooids" has been applied.
As regards the development of the Calycophoridae, "not only the new polypites, but the new nectocalyces and reproductive organs, and even the branches of the tentacles, are developed on the proximal side of the old ones; so that the distal appendages are the oldest" (Huxley). The process of development is therefore the reverse of what obtains amongst the Hydroida.
Diphyes (fig. 55), which may be taken as the type of the Calycophoridae, consists of a delicate filiform coenosarc, provided proximally with two large mitre-shaped nectocalyces (v, v'), of which one lies entirely on the distal side of the other. The pointed apex of the distal nectocalyx is received into a special cavity in the proximal nectocalyx. The "hydrcecium" (h) is formed partially by this chamber in the nectocalyx, and partially by an arched groove prolonged upon the inner surface of the distal nectocalyx, within which the coenosarc moves freely up and down, and can be entirely retracted if necessary. The upper part of the coenosarc dilates into a small ciliated cavity, from which are given off two tubes, which proceed respectively to the distal and proximal necto-calyces, where they open into the central chamber from which the nectocalycine canals take their rise. The upper portion of this small ciliated cavity is prolonged proximally into the larger chamber of the "somatocyst." The coenosarc (c) bears polypites, each of which is protected by a delicate glassy "hydrophyllium."