Limnocodium sowerbyi was first discovered in the Victoria regia tank in the Botanic Gardens, Regent’s Park, London. Since then it has been discovered in other botanic gardens in various parts of Europe, its two most recent appearances being at Lyons (1901) and Munich (1905), occurring always in tanks in which the Victoria regia is cultivated, a fact which indicates that tropical South America is its original habitat. In the same tanks a small hydroid, very similar to Microhydra, has been found, which bears medusa-buds and is probably the stock from which the medusa is budded. It is a remarkable fact that all specimens of Limnocodium hitherto seen have been males; it may be inferred from this either that only one polyp-stock has been introduced into Europe, from which all the medusae seen hitherto have been budded, or perhaps that the female medusa is a sessile gonophore, as in Pennaria. The male gonads are carried on the radial canals.
Limnocnida tanganyicae was discovered first in Lake Tanganyika, but has since been discovered also in Lake Victoria and in the river Niger. It differs from Limnocodium in having practically no manubrium but a wide mouth two-thirds the diameter of the umbrella across. It buds medusae from the margin of the mouth in May and June, and in August and September the gonads are formed in the place where the buds arose. The hydroid phase, if any, is not known.
Both these medusae have sense-organs of a peculiar type, which are said to contain an endodermal axis like the sense-organs of Trachylinae, but the fact has recently been called in question for Limnocodium by S. Goto, who considers the genus to be allied to Olindias. Allman, on the other hand, referred Limnocodium to the Leptomedusae.
In this connexion must be mentioned, finally, the medusae budded from the fresh-water polyp Microhydra. The polyp-stages of Limnocodium and Microhydra are extremely similar in character. In both cases the hydranth is extremely reduced and has no tentacles, and the polyp forms a colony by budding from the base. In Limnocodium the body secretes a gelatinous mucus to which adhere particles of mud, &c., forming a protective covering. In Microhydra no such protecting case is formed. In view of the great resemblance between Microhydra and the polyp of Limnocodium, it might be expected that the medusae to which they give origin would also be similar. As yet, however, the medusa of Microhydra has only been seen in an immature condition, but it shows some well-marked differences from Limnocodium, especially in the structure of the tentacles, which furnish useful characters for distinguishing species amongst medusae. The possession of a polyp-stage by Limnocodium and Microhydra furnishes an argument against placing them in the Trachylinae. Their sense-organs require renewed investigations. (Browne [10] and [10a].)
Order VI. Siphonophora.—Pelagic floating Hydrozoa with great differentiation of parts, each performing a special function; generally regarded as colonies showing differentiation of individuals in correspondence with a physiological division of labour.
| Fig. 68.—Diagram showing possible modifications of medusiform and hydriform persons of a colony of Siphonophora. The thick black line represents endoderm, the thinner line ectoderm. (After Allman.) |
| n, Pneumatocyst. k, Nectocalyces (swimming bells). l, Hydrophyllium (covering-piece). i, Generative medusiform person. g, Palpon with attached palpacle, h. e, Siphon with branched grappling tentacle, f. m, Stem. |
A typical Siphonophore is a stock or cormus consisting of a number of appendages placed in organic connexion with one another by means of a coenosarc. The coenosarc does not differ in structure from that already described in colonial Hydrozoa. It consists of a hollow tube, or tubes, of which the wall is made up of the two body-layers, ectoderm and endoderm, and the cavity is a continuation of the digestive cavities of the nutritive and other appendages, i.e. of the coelenteron. The coenosarc may consist of a single elongated tube or stolon, forming the stem or axis of the cormus on which, usually, the appendages are arranged in groups termed cormidia; or it may take the form of a compact mass of ramifying, anastomosing tubes, in which case the cormus as a whole has a compact form and cormidia are not distinguishable. In the Disconectae the coenosarc forms a spongy mass, the “centradenia,” which is partly hepatic in function, forming the so-called liver, and partly excretory.
The appendages show various types of form and structure corresponding to different functions. The cormus is always differentiated into two parts; an upper portion termed the nectosome, in which the appendages are locomotor or hydrostatic in function, that is to say, serve for swimming or floating; and a lower portion termed the siphosome, bearing appendages which are nutritive, reproductive or simply protective in function.
Divergent views have been held by different authors both as regards the nature of the cormus as a whole, and as regards the homologies of the different types of appendages borne by it.
The general theories of Siphonophoran morphology are discussed below, but in enumerating the various types of appendages it is convenient to discuss their morphological interpretation at the same time.