Fig. 43. - Diagram of sporosacs supported upon a gonoblastidion (or blastostyle). a Chitinous investment (periderm) of the colony; b Ectoderm; c Endoderm ; p Poly-pite; g Gonoblastidion, or columniform zooid, carrying sporosacs (s s) with ova in their interior. (Altered from Allman.)
In some cases it is formed by the hollowing out of the original sphere, and the formation of an opening (the primitive mouth) at one end, as seems to be generally the case in the Coelenterata; or, in other cases, it may be produced by an invagination or inversion of the primitive vesicle in such a manner as to form a central chamber, with a single aperture opening on the exterior. By fixation of the "gastrula" at its hinder extremity to some foreign object, and by the formation of tentacles round the mouth-opening at the other extremity, a hydraform polypite is at once produced, which (if not belonging to one of the simple forms) proceeds to develop the composite adult by a process of gemmation. In this process in the Corynida (as also in the Sertularida and Campanularida) the new polypites are developed at or near the distal end of the hydrosoma, the distal polypites being thus the youngest; whereas the reverse of this obtains amongst the Oceanic Hydrozoa.
The subject of the reproduction of the Corynida having been treated at some length, so as to apply to the remaining Hydroida, we shall now give a brief description of the leading types of structure exhibited by the order.
Eudendrium, a genus of the Corynida, which is not uncommonly found attached to submarine objects, usually in tolerably deep water, may be taken as a good example of the fixed and composite division of the order. The hydrosoma consists of numerous polypites, united by a coenosarc, which is more or less branched, and is defended by a horny tubular poly-pary. The polypites are borne at the ends of the branches and branchlets, and are not contained in "hydrothecae," the polypary ending abruptly at their bases. The polypites are non-retractile, of a reddish colour, and provided with about twenty tentacles, arranged round the mouth in a single row. Tubularia (fig. 44) is very similar to Eudendrium, but the hydrosoma is either undivided or is very slightly branched. The hydrosoma consists of clustered horny tubes, of a straw colour, and not unlike straws to look at; hence the common name of pipe-coralline given to this zoophyte. Each tube is filled with a soft, semifluid, reddish coenosarc, and gives exit at its distal extremity to a single poly-pite. The polypites are bright red in colour, and are not retractile within their tubes, the horny polypary extending only to their bases. The polypites are somewhat conical in shape, the mouth being placed at the apex of the cone, and they are furnished with two sets of tentacles. One set consists 'of numerous short tentacles placed directly round the mouth ; the other is composed of from thirty to forty tentacles of much greater length, arising from the polypite about its middle or near the base. Near the insertion of these tentacles the generative buds are produced at proper seasons. The generative buds remain permanently attached, but each is furnished with a swimming-bell, in which canals are present. The manubrium is destitute of a mouth, and "the swimming-bell is converted into a nursery in which the embryo passes through the later stages of its development" (Hincks).
Coryomorpha nutans may be taken to represent those Corynida in which there is no polypary and the hydrosoma is simple. It is about four inches in length, and is fixed by filamentous roots to the sand at the bottom of the sea. It consists of a single whitish polypite, striped with pink, and terminating upwards in a spear-shaped head, round the thickest part of which is a circlet of from forty to more than one hundred long white tentacles. Above these comes a series of long, branching gonoblastidia, bearing gono-phores, and succeeded by a second shorter set of tentacles which surround the mouth. The gonophores become ultimately detached as free-swimming medusoids.
Another remarkable example of the Corynida is Hydradinia (fig. 45). In this genus the polypites are gregarious, and the polypary forms a horny crust which spreads over shells and other foreign bodies. The tentacles of the nutritive zooids form a single sub-alternate series. The generative buds are produced upon imperfect, non-tentaculate polypites ; and are mere sac-shaped protuberances, enclosing diverticula from the body-cavity, but not detached from the parent organism.
Fig. 44. - Corynida. Fragment of Tubularia indivisa, natural size.
Fig. 45. - Group of zooids of Hydractinia echinata, enlarged. (After Hincks.) a a Nutritive zooids ; b b Generative zooids, carrying sacs filled with ova.