Fig. 141.—A small Crustacean (Rhinocalanus) caught by a terminal filament (f.t) of a battery of Stephanophyes. b, The proximal end of the battery with the most powerful nematocysts; e, elastic band; S, stalk supporting the battery on the dactylozooid. (After Chun.)
The dactylozooid in these forms has a hollow axis, and the lumen is continuous with the cavity of the neighbouring gastrozooid. Arranged at regular intervals on the axis is a series of tentacles ("tentilla"), and each of these supports a kidney-shaped swelling, the "cnidosac," or battery, which is sometimes protected by a hood. Each battery contains an enormous number of nematocysts. In Stephanophyes, for example, there are about 1700 nematocysts of four different kinds in each battery. At the extremity of the battery there is a delicate terminal filament. The action of the battery in Stephanophyes is, according to Chun,[[334]] a very complicated one. The terminal filament lassos the prey and discharges its somewhat feeble nematocysts at it (Fig. 141). If this kills it, the dactylozooid contracts and passes the prey to a gastrozooid. If the animal continues its struggles, it is drawn up to the distal end of the battery and receives the discharge of a large number of nematocysts; and if this also fails to put an end to its life, a membrane covering the largest and most powerful nematocysts at the proximal end of the whole battery is ruptured, and a final broadside of stinging threads is shot at it.
The larger nematocysts of these batteries in the Siphonophora are among the largest found in Coelenterata, being from 0.5 to 0.1 mm. in length, and they are frequently capable of inflicting painful stings on the human skin. The species of Physalia, commonly called "Portuguese Men-of-War," have perhaps the worst reputation in this respect, the pain being not only intense but lasting a long time.
Hydrophyllia.—In many Siphonophora a number of short, mouthless, non-sexual zooids occur, which appear to have no other function than that of shielding or protecting other and more vital parts of the colony. They consist of an axis of firm mesogloea, covered by a layer of flattened ectoderm, and they may be finger-shaped or triangular in form. In Agalma and Praya an endoderm canal perforates the mesogloea and terminates in a little mouth at the free extremity. In Athoria and Rhodophysa the hydrophyllium terminates in a little nectocalyx.
Pneumatophore.—In all the Siphonophora, with the exception of the Calycophorae, there is found on one side or at one extremity of the colony a vesicle or bladder containing a gas,[[335]] which serves as a float to support the colony in the water. This bladder or pneumatophore is probably in all cases a much modified nectocalyx. It shows great variations in size and structure in the group. It is sometimes relatively very large, as in Physalia and Velella, sometimes very small, as in Physophora. It is provided with an apical pore in some genera (Rhizophysa), or a basal pore in others (Auronectidae), but it is generally closed. In the many chambered pneumatophore of the Chondrophoridae there are several pores.
In many forms two distinct parts of the pneumatophore can be recognised—a distal region lined by chitin,[[336]] probably representing the sub-umbrellar cavity of the nectocalyx, and a small funnel-shaped region lined by an epithelium, the homology of which is a matter of dispute. It is believed that the gas is secreted by this epithelium. In the Auronectidae the region with secretory epithelium is relatively large and of a more complicated histological character. It is remarkable also that in this family the pore communicates, not with the chitin-lined region, but directly with the epithelium-lined region.
There is no pneumatophore in the Calycophorae, but in this sub-order a diverticulum of an endoderm canal secretes a globule of oil which may serve the same hydrostatic function.
The stolon is the common stem which supports the different zooids of the colony. In the Calycophorae the stolon is a long, delicate, and extremely contractile thread attached at one end to a nectocalyx, and bearing the zooids in discontinuous groups. These groups of zooids arranged at intervals on the stolon are called the "cormidia." The stolon is a tube with very thick walls. Its lumen is lined by a ciliated endoderm with circular muscular processes, and the surface is covered with an ectoderm, also provided with circular muscular processes. Between these two layers there is a relatively thick mesogloea showing on the outer side deep and compound folds and grooves supporting an elaborate system of longitudinal muscular fibres. In many Physonectidae the stolon is long and filamentous, but not so contractile as it is in Calycophorae, but in others it is much reduced in length and relatively stouter. The reduction in length of the stolon is accompanied by a complication of structure, the simple tubular condition being replaced by a spongy complex of tubes covered by a common sheath of ectoderm. In the Auronectidae the stolon is represented by a conical or hemispherical spongy mass bearing the zooids, and in the Rhizophysaliidae and Chondrophoridae it becomes a disc or ribbon-shaped pad spreading over the under side of the pneumatophore.
Gonozooids.—The gonozooids are simple tubular processes attached to the stolon which bear the Medusae or the degenerate medusiform gonophores. In the Chondrophoridae the gonozooids possess a mouth, but in most Siphonophora they have neither mouth nor tentacles. In some cases, such as Anthophysa, the colonies are bisexual—the male and female gonophores being borne by separate gonozooids—but in others (e.g. Physalia) the colonies appear to be unisexual.