In certain Corynida, however, we meet with a still higher form of structure, the gonophores being now said to be "medusoid." In these cases the generative bud is primitively a simple sac - such as the "sporosac" - but ultimately develops itself into a much more complicated structure. The gonophore (fig. 41, c) is now found to be composed of a bell-shaped disc, termed the "gonocalyx," which is attached by its base to the parent organism (the trophosome), and has its cavity turned outwards. From the roof of the gonocalyx, like the clapper of a bell, there depends a peduncle or "manubrium," which contains a process of the somatic cavity. The manubrium gives out at its fixed or proximal end four prolongations of its cavity, in the form of radiating lateral tubes which run to the margin of the bell, where they communicate with one another by means of a single circular canal which surrounds the mouth of the bell. This system of tubes constitutes what is known as the system of the "gastro-vascular" or "gonocalycine canals." The gonophore, thus constituted, may remain permanently attached to the parent organism, as in Tubularia indivisa (fig. 41, c); but in other cases still further changes ensue. In the higher forms of development (fig. 42) the manubrium acquires a mouth at its free or distal extremity, and the gonocalyx becomes detached from the parent. The gonophore is now free, and behaves in every respect as an independent being. The gonocalyx is provided with marginal tentacles and with an inward prolongation from its margin, which partially closes the mouth of the bell, and is termed the "veil" or "velum." By the contractions of the gonocalyx, which now serves as a natatorial organ, the gonophore is propelled through the water. The manubrium, with the shape, assumes the functions of a polypite, and its cavity takes upon itself the office of a digestive sac. Growth is rapid, and the gonophore may attain a comparatively gigantic size, being now absolutely identical with one of those organisms which are commonly called "jelly-fishes," and are technically known as Medusae (fig. 42). In fact, as we shall afterwards see, many of the gymnophthalmate Medusae, originally described as a distinct order of free-swimming Hydrozoa, are in truth merely the liberated generative buds, or "medusiform gonophores," of the permanently rooted Hydroids. Finally, the essential generative elements - the ova and spermatozoa - are developed in the walls of the manubrial sac, between its endoderm and ectoderm, and embryos are produced. These embryos, however, instead of resembling the organism which immediately gave them birth, develop themselves into the fixed Corynid from which the gonophore was produced, thus completing the cycle. The swimming-bell of the medusiform gonophore is believed to be formed by a great development of an inter-tentacular web, such as is sometimes present, in a rudimentary form, in the nutritive zooids. Sometimes the medusoid becomes quiescent towards the close of its existence, and the swimming-bell becomes reversed or atrophied. Lastly, in Clavatella, the sexual zooid, though free and locomotive, is not provided with a swimming-bell, but creeps about by means of suctorial discs developed on branches of the tentacles.
Fig. 42. - Free-swimming medusiform gonophore of Bougainvillea superciliaris a fixed Hydroid. Enlarged. (After A. Agassiz.)
As we have seen, the generative buds of the Corynida may exist in the following chief forms : 1. As "sporosacs," or simple closed sacs, consisting of ectoderm and endoderm, with a central cavity in which ova and spermatozoa are produced. 2. As "disguised medusoids," in which there is a central manu-brial process and a rudimentary system of gonocalycine canals; but the gonocalyx remains closed. 3. As complete medusoids, which have a central manubrium, a complete system of gonocalycine canals, and an open gonocalyx; but which never become detached. 4. As perfect medusiform gonophores (fig. 42), which are detached, and lead an independent existence for a time, until the generative elements are matured. In whichever of these forms the gonophore may be present, the place of its origin from the trophosome may vary in different species of the order. 1. They may arise from the sides of the polypites, as in Coryne and Stauridia; 2. They may be produced from the coenosarc, as in Cordylophora ; 3. They may be produced upon certain special processes, which are termed "gono-blastidia," as in Hydractinia and Dicoryne. These gonoblastidia (fig. 43, g) are processes from the body-wall or coenosarc, which closely resemble true polypites in form, but differ from them in being usually devoid of a mouth, and in having shorter tentacles. They are, in truth, atrophied or undeveloped polypites.
The gonoblastidia are the "blastostyles" of Prof. Allman, and are usually columniform in shape. They may carry sporosacs, or medusoid gonophores; and they may be naked, or, in other orders, they may be protected within a chitinous receptacle or "gonangium."
As regards the development of the Corynida, the embryo is very generally, though not always, ciliated at first, when it is known as a "planula;" but in one form the embryo leaves the gonophore as a free and locomotive polypite, and in another it is non-ciliated and amoeboid. The "planula" is a minute ciliated cylindrical body, which swims about actively in the water. The embryonic cells of which it is composed divide into an outer and an inner layer, enclosing a central cavity, and it next passes into a condition which is common to the embryos of the Coelenterata generally, and to which Haeckel has applied the name of "gastrula." At this stage, it consists of an ovate or rounded body, with a single central cavity, which communicates with the exterior by an aperture placed at one pole. The wall of this central cavity consists of two layers, an outer and an inner, corresponding with the ectoderm and endoderm of the adult, and also with the two primitive layers of the germ of the Vertebrata. The "gastrula" stage appears to be one very generally passed through by all animals higher than the Protozoa, and by the Sponges amongst the latter, but there is a difference as to the manner in which the central cavity is formed.