The order Corynida comprises those Hydrozoa whose hydrosoma is fixed by a hydrorhiza, and consists either of a single polypite, or of several united by a coenosarc, which usually develops a firm outer layer or "polypary" No "hydrothecae" are present. "The reproductive organs are in the form of gono-phores, which vary much in structure, and arise from the sides of the polypites, from the coenosarc, or from gonoblastidia" (Greene). . The hydrosoma of the Corynida may consist of a single polypite, as in Coryomorpha and Vorticlava, or it may be composed of several united by a coenosarc, as in Cordylophora (fig. 39, a). The order is entirely confined to the sea, with the single exception of Cordylophora, which inhabits fresh water. In Tubularia and its allies the organism is protected by a well-developed external chitinous envelope or "polypary;" but in the other genera belonging to the order, the polypary is either rudimentary or is entirely absent. The polypary of the Corynida, when present, is readily distinguished from that of the Sertularida, by the fact that in the former it extends only to the bases of the polypites; whereas in the latter it expands to form little cups for the reception of the polypites, these cups being called "hydrothecae." Owing to the fact that neither the polypites nor the generative buds of the Corynida are enclosed in a chitinous investment, the name of "Gymnoblastic Hydroids" is applied to them by Professor Allman.
Fig. 39. - Morphology of Corynida. a Fragment of Cordylophora lacustris, slightly enlarged; b Fragment of the same considerably enlarged, showing a polypite and three gonophores in different stages of growth, the largest containing ova ; c Portion . of Syncoryne Sarsii with medusiform zooids budding from between the tentacles.
As regards the reproductive process in the Corynida, the reproductive elements are developed in distinct buds or sacs, which are external processes of the body-wall, and have been aptly termed "gonophores" by Professor Allman. Strictly speaking, Dr Allman understands by the term "gonophore" only the ultimate generative zooid, that which immediately produces the generative elements.* Great variations exist in the form and development of these generative buds, and an examination of these leads us to some of the most singular phenomena in the entire animal kingdom. In some species of Hydractinia and Coryne, the generative buds or "gonophores" exist in their simplest form - namely, as sacciform protuberances of the endoderm and ectoderm, enclosing a diverticulum of the somatic cavity. In this form they are attached to the "trophosome" by a short stalk, and they are termed "sporo-sacs" (fig. 40). They are exactly like the buds which we have already seen to exist in the Hy-dra, with this difference, that they are not themselves developed into fresh polypites, but are simply receptacles in which the essential elements of generation - the ova and spermatozoa - are prepared, by the union of which the young Corynid is produced. The sporosac is almost invariably permanently attached to the trophosome, the only known exception being in Dicoryne, in which the sporosac, previous to the discharge of its ova, liberates itself from its outer investment, and swims about freely as an independent ciliated organism.
Fig. 40. - Sporosac of Hydractinia echin-ata (after Allman). a Outer wall of the sac ; b inner wall of the sac; s Column developed from the floor of the sporosac, and extending into its cavity. This is termed the "spadix;" it contains a prolongation from the coeno-sarcal canal, and the ova are developed around it.
* According to Mr Hincks, the "gonophore" is the bud in which the reproductive elements are formed. "It consists of an external envelope (ectotheca), enclosing either a fixed generative sac between the walls of which the ova and spermatozoa are developed, or a free sexual zooid." The actual sexual zooid is termed by Mr Hincks the "gonozooid," whether it be fixed or free - in other words, it is the gonophore minus its external investment. The gonozooid is sometimes male, sometimes female ; and the same colony may produce one or both - the former being most commonly the case.
In Cordylophora (fig. 41, b) a further advance in structure is perceptible. The gonophore now consists of a closed sac, from the roof of which depends a hollow process or peduncle - the "manubrium" - which gives off a system of tubes which run in the walls of the sac. For reasons which will be immediately evident, the gonophore in this case is said to have a "disguised" medusoid structure (fig. 41, b).
Fig. 41. - Reproductive processes of Hydrozoa. a Sporosac ; b Disguised medusoid ; c Attached medusiform gonophore; d Free medusiform gonophore. The cross shading indicates the reproductive organs, ovaria or spermaria. The part completely black indicates the cavity of the manubrium and the gonocalycine canals.