Order I. Eleutheroblastea. " II. Hydroidea (_Leptolinae_). Sub-order 1. Gymnoblastea (_Anthomedusae_). " 2. Calyptoblastea (_Leptomedusae_). Order III. Hydrocorallinae. " IV. Graptolitoidea. " V. Trachylinae. Sub-order 1. _Trachomedusae_. " 2. _Narcomedusae_. Order VI. Siphonophora. Sub-order 1. Chondrophorida. " 2. Calycophorida. " 3. Physophorida. " 4. Cystophorida. _Organization and Morphology of the Hydromedusae._ As already stated, there occur in the Hydromedusae two distinct types of person, the polyp and the medusa; and either of them is capable of non-sexual reproduction by budding, a process which may lead to the formation of colonies, composed of more or fewer individuals combined and connected together. The morphology of the group thus falls naturally into four sections--(1) the hydropolyp, (2) the polyp-colony, (3) the hydromedusa, (4) the medusa-colonies. Since, however, medusa-colonies occur only in one group, the Siphonophora, and divergent views are held with regard to the morphological interpretation of the members of a siphonophore, only the first three of the above subdivisions of hydromedusa morphology will be dealt with here in a general way, and the morphology of the Siphonophora will be considered under the heading of the group itself. [Illustration: FIG. 1.--Diagram of a typical Hydropolyp. a, Hydranth; b, Hydrocaulus; c, Hydrorhiza; t, Tentacle; ps, Perisarc, forming in the region of the hydranth a cup or hydrotheca (h, t),--which, however, is only found in polyps of the order Calyptoblastea.]