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. [Illustration: After A. Agassiz, from Lankester's _Treatise on Zoology_. FIG. 69.--_Porpita_, seen from above, showing the pneumatophore and expanded palpons.] In the nectosome one or more of the following types of appendage occur:-- 1. Swimming-bells, termed _nectocalyces_ or _nectophores_ (fig. 68, k), absent in _Chondrophorida_ and _Cystophorida_; they are contractile and resemble, both in appearance, structure and function, the umbrella of a medusa, with radial canals, ring-canal and velum; but they are without manubrium, tentacles or sense-organs, and are always bilaterally symmetrical, a peculiarity of form related with the fact that they are attached on one side to the stem. A given cormus may bear one or several nectocalyces, and by their contractions they propel the colony slowly along, like so many medusae harnessed together. In cases where the cormus has no pneumatophore the topmost swimming bell may contain an oil-reservoir or _oleocyst_.