Order IX. Siphonophora.
In this order the naturalist finds collected together a number of very beautiful, delicate transparent organisms to which the general term "jelly-fish" may be applied, although their organisation is far more complicated and difficult to describe than that of any of the Medusae. In several of the Hydrozoa the phenomenon of dimorphism has already been noticed. In these cases one set of individuals in a colony performs functions of stinging and catching food and another the functions of devouring and digesting it. In many of the Siphonophora there appears to be a colony of individuals in which the division of labour is carried to a much further extent than it is in the dimorphic Hydrozoa referred to above. Not only are there specialised gastrozooids and dactylozooids, but also gonozooids, zooids for propelling the colony through the water ("nectocalyces"), protective zooids ("hydrophyllia"), and in some cases a specialised zooid for hydrostatic functions; the whole forming a swimming or floating polymorphic colony. But this conception of the construction of the Siphonophora is not the only one that has met with support. By some zoologists the Siphonophoran body is regarded not as a colony of individuals, but as a single individual in which the various organs have become multiplied and dislocated.
The multiplication or repetition of organs that are usually single in each individual is not unknown in other Hydrozoa. In the Medusa of the Gymnoblast Syncoryne, usually known as Sarsia, for example, there is sometimes a remarkable proliferation of the manubrium, and specimens have been found with three or four long manubria attached by a tubular stalk to the centre of the umbrella. Moreover, this complex of manubria may become detached from the umbrella and live for a considerable time an independent existence.[[333]]
If we regard the manubrium of a Medusa as an organ of the animal's body, it might be thought obvious that the phenomenon observed in the Medusae of Syncoryne is a case of a simple repetition of the parts of an individual; but the power that the group of manubria possesses of leading an independent existence renders its interpretation as a group of organs a matter of some inconvenience. If we can conceive the idea that an organ may become detached and lead an independent existence, there is no reason why we should not regard the Medusa itself of Syncoryne as an organ, and we should be driven to the paradoxical conclusion that, as regards several genera and families of Hydrozoa, we know nothing at present of the individuals, but only of their free-swimming organs, and that in others the individual has degenerated, although one of its organs remains.
There is, however, no convincing argument to support either the conception that the Siphonophoran body is a colony of individuals, or that it is an individual with disjointed organs. These two conceptions are sometimes called the "Poly-person" and "Poly-organ" theories respectively. The difficulty is caused by the impossibility of giving any satisfactory definition in the case of the Hydrozoa of the biological terms "organ" and "individual." In the higher animals, where the correlation of parts is far more complex and essential than it is in Coelenterata, a defined limit to the scope of these terms can be laid down, but in the lower animals the conception of what is termed an organ merges into that which is called an individual, and no definite boundary line between the two exists in Nature. The difficulty is therefore a permanent one, and, in using the expression "colony" for the Siphonophoran body, it must be understood that it is used for convenience' sake rather than because it represents the only correct conception of the organisation of these remarkable Coelenterates.
Regarding the Siphonophora as polymorphic colonies, then, the following forms of zooids may be found.
Nectocalyces.—The nectocalyces are in the form of the umbrella of a medusa attached to the stolon of the colony by the aboral pole. They are provided with a velum and, usually, four radial canals and a circular canal. There is no manubrium, and the marginal tentacles and sense-organs are rudimentary or absent. There may be one or more nectocalyces in each colony, and their function is, by rhythmic contractions, to propel the colony through the water (Fig. 142, N).
Gastrozooids.—These are tubular or saccular zooids provided with a mouth and attached by their aboral extremity to the stolon (Fig. 142, G). In some cases the aboral region of the zooid is differentiated as a stomach. It is dilated and bears the digestive cells, the oral extremity or hypostome being narrower and more transparent. In some cases the mouth is a simple round aperture at the extremity of the hypostome, but in others it is dilated to form a trumpet-like lip.
Dactylozooids.—In Velella and Porpita the dactylozooids are similar in general characters to the tentacles of many Medusae. They are arranged as a frill round the margin of the colony, and each consists of a simple tube of ectoderm and endoderm terminating in a knobbed extremity richly provided with nematocysts.
In many other Siphonophora, however, the dactylozooids are very long and elaborate filaments, which extend for a great distance from the colony into the sea. They reach their most elaborate condition in the Calycophorae.