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. [Illustration: 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.]