The sexual zooid is very commonly degenerate, and is always so in the two Campanularian families Plumularidae and Serhilaridae1, and the following stages may be enumerated (Weissman): (i) Medusoid. There are no tentacles, and as a rule no velum, no organs of special sense, nor mouth to the manubrium, detached when ripe, e.g. Pennaria2. (2) Sessile Medusoid. The bell has either incomplete canals or none at all, but there is a bell-mouth and cavity, e. g. Tubularia, female Cladocoryne. (3) Sessile Gono-phore. Bell devoid of cavity and mouth, but with a gastral lamella, subum-brellar and manubrial ectoderm laminae, e. g. Clava, Hydractinia, Plurnu-laria. (4) Sessile Gonophore. Bell with gastral lamella, subumbrellar and manubrial ectoderm forming a single lamina if present: female Campanula-ria, Opercularella, Halecium. (5) Sporosac= Sporophore. A sessile gonophore without trace of medusoid structure, e.g. Cordylopkora, male Campanidaria3. The term 'spadix' is applied to the central closed endodermic structure representing the manubrial cavity in a gonophore or sporosac.
It is sometimes branched, e. g. in Cordylopkora. An alteration in the place and mode of origin of the generative products usually accompanies these changes, but not always to the same degree in the two sexes: see pp. 767-8, post.
As to the Hydrocorallina, the generative products of Millepora are de-veloped in small capsules in the course of the coenosarcal canals; of Sty-lasteridae in a sporosac formed in the course of the same canals and lodged in an ampulla or cavity of the coenosteum4.
Asexual reproduction takes place by fission or gemmation. Fission is rare. It occurs in the hydroid Protohydra Leuckarti, where it is binary and transverse; in Polypodium, where it is longitudinal (infra, p. 766); and in certain Leptomedusans, Stomobrachium mirabile ( = a young form of Mesonema coerulescens), in Phialidium variabile, and Gastroblasta Raffaelei.
1The medusae produced by Syncoryne (Coryne) mirabilis are said by L. Agassiz to be set free in March, but in April to be sessile and more or less arrested in development; cf. Allman, op. cit. ante, p. 278, and lit. cited. Campanularia volubilis is said by Du Plessis to bear medusae in summer, gonophores in winter (A. N. 41 (2), p. 412). See also Wagner, Wirbellosen des Weissen Meeres, Leipzig, 1885, p. 78.
2The Meconidium of Gonothyrea is a medusoid which is not detached; and the male is more arrested than the female. See Weismann, op. cit. ante, pp. 135-7, 139 and Allman, op. cit. ante, pp. 55-8, Fig. 28.
3The Tubularian Dicoryne conferta has a remarkable free swimming ciliated sporosac with two solid tentacles attached to the base of the spadix. The spermatozoa surround the spadix, but in the female there are only two ova, one on either side of it. See Allman, op. cit. ante, p. 226. In Sertularella polyzonias the gonophore is suppressed and the genital products ripen in the walls of a blastostyle: see Weismann, op. cit. p. 166, note p. 252, p. 265. On the nature of the blastostyle cf P. 757, ante, and note 2.
The development of a sporosac after discharging the generative products into a hydranth has been described in Cordylophora by Allman, and L. Agassiz has observed a similar case in Rhizogeton: cf. Allman, op. cit. p. 204. Some doubt is perhaps permissible on the subject.
4Quelch states that there are ampullae in Millepora Murrayi: see Reef Corals, Challenger Reports, xvi. p. 192; or Nature, xxx. p. 539.
In the first-named the manubrium divides, and then the bell. In Phialidium a second manubrium appears as a bud at the base of the first, and fission then takes place between the two. A third may be similarly formed in the course of a radial canal. Gastroblasta has the number of rrranubria increasing progressively by budding on the radial canals, and fission occurs between the oldest and second oldest. Sexual products were observed in the two last-named1. The detachment by fission of the end of a growing branch has been observed in some Campanularidae: and in one instance the fixation of the detached portion, the formation of a hydranth, and subsequently of a colony 2. Gemmation is universal in the hydroid form, Protohydra only excepted: and is not uncommon in Medusae, for the most part belonging to the Anthomedusae. In Hydra, Microhydra, and Tiarella the hydroid form thus produced is set free, but the first-named may be temporarily colonial, the individual not only producing more than one bud, but its buds other buds in their turn before detachment. In all other instances the hydroid buds remain attached to one another, constituting a colony. They originate from the hydrocope of the first hydranth, and afterwards from the coenosarc in a definite manner.
The buds in a hydroid colony which develope into sexual zooids may take origin from the hydrorhiza when the hydranths do so, from the branches of a colony, then occupying either the same or a different position to a hydranth, from the peduncle of a normal hydranth, from its hydrocephalis, from a blastostyle, or from a special peduncle of unknown significance (Tubularia, Corymorpha, Monocauhts) 3. The buds borne by a Medusa invariably develope into a Medusa: they may be situated on the manubrium, on the tentacles or at their bases, on the radial canals, or circumferential canal, and gemmation may be continued through more than one generation.
The sexes are as a rule separate. Hydra and Eleutheria are hermaphrodite: abortive ova occur in the male of Gonothyrea Loveni, Male and female gonophores grow on the same blastostyle in Myriothela, on the same stem of certain Sertularians, or on stems of the same colony in the Tubularian Dicoryne. The ovary may consist of but one ovum, e. g. in Eudendrium, Campamdaria flexuosa; or of many, and in the latter case a certain proportion of the ova, e. g. in Coryne pusilla, Tubularia mesem-bryanthemum, Plumularia Halecioides, or all save one, e. g. in Hydra, Tubidaria indivisa, Myriothela, undergo atrophy, and the products to which they give origin serve as food-material to the remainder. A vitelline membrane is absent in all ova borne by a Medusa, and as a rule when development takes place within a gonophore or in an acrocyst. When present it may be delicate, thickened, or shell-like as in Corydendrium, Pennaria, Eudendrium capillare. Impregnation and segmentation take place in Hydra before the ovum is laid. Development also frequently occurs within the gonophore, e. g. in Cordylophora lacustris, Campamdaria flexuosa, Sty-lasteridae, etc.