With the degeneration in development of the sexual zooid (p. 762, ante) the place in which the sexual cells first originate is altered. They may arise in the entocodon, e. g. in Tubularia (2), male Clava (3), Pennaria (1)2; in the endoderm of the gonophoral bud, e. g. Coryne (3); or in the coenosarc, but in different positions, e. g. at the place of origin of the gonophore itself, Hydractinia (3), male Cordylophora (5), female Clava (3), in the budding zone of the hydranth, from the hydrocope of which the blastostyle buds, male Eudendrium racemosum (3), Gonothyrea (1), Campanularia (female 4, male 5), or in a budding zone of a principal hydranth of the stem, female Cordylophora (5), and Eudendrium (5). They sometimes originate in the hydrorhiza, as in Eudendrium capillare (5) and Eucopella (1). The sexual cells themselves in these cases are very commonly cells of the ectoderm, and they may migrate in it from one spot to another, e. g. female Cordylophora, male Campanularia flexuosa. They may pass into the endoderm, wander through it, and pass back into the ectoderm, where they ripen as in the female Eudendrium racemosum.
But they very frequently appear first in the endo-derm, and wander through it to the place where they ripen in the ectoderm, e. g. female Clava and Podocoryne, male Eudendrium racemosum, E. capillare, Gono-thyrea Loveni, female Campanularia flexuosa, Eucopella, Halecium, Sertularia pumila, Plumularia echinulata, P. (Anisicola) Halecioides, Antennularia antennina, Aglaophenia pluma. It is rare for the sexual cells to ripen in the endoderm as in Corydendrium parasiticum and the male Pachycordyle Neapolitana. It is uncertain whether or no they ever originate really in any Hydroidean from endoderm cells. Such an origin has been asserted in Eucopella by von Lendenfeld; in Plumularia fragilis by Hamann (J. Z. xv. p. 501); in Campanularia flexuosa by Fraipont and de Varenne; but in the last-named case it is doubted by Weismann (Entstehung, etc. p. 147), as it is in Gonothyrea (p. 133). The young cells lying in the endoderm may be immigrant ectoderm cells.
1A partial reversion has been noted by Allman in a Medusa, Circe invertens, Nature, ix. p. 74; see p. 760, and note p. 751.
2The numbers in brackets correspond to the numbers affixed to the paragraphs on p. 762, and how what stage of degeneration the sexual zooid has reached.
The forms of sexual zooids 1-4, p. 762, are to be regarded as degenerate medusae, not as forms tending to acquire a medusan structure. It is difficult to explain the presence of rudimentary and peculiar medusan organs, e. g. entocodon, gastral lamella, on any other hypothesis (cf. Weismann, Entstehung, p. 255), particularly when coupled with the remarkable facts above detailed concerning the sexual cells. Moreover, Hydroids obviously genetically related, may vary extremely in the character of their sexual zooids, e. g. Podocoryne has free medusae; Hydractinia sessile gonophores with gastral lamella, etc.; and in Oorhiza the female sporosacs, the only ones known, are seated in groups on the hydrorhiza, and have apparently a very simple structure (Mereschkowsky, A. N. H. (5), i. 1878, p. 325; Wagner, Wirbellosen des Weissen Meeres, p. 71); or again Coryne with sessile gonophores, Syncoryne with free medusae. Variations of this character and the mode of origin of the sexual cells, prove also that the sporosac (5, p. 762) is a degenerate medusa, at least in the Campanulariae. And Weismann concludes that the same is the case with the sporosac of the Tubularians (cf. Entstehung, pp. 244-252 and p. 291). Their frequent origin from a blastostyle favours the same view (op. cit. pp. 246-7). The physiological explanation of these changes is probably to be found in an earlier ripening of the genital products, a process benefiting the race and lessening the danger of its extinction (Weismann, pp. 262-65). See on the whole subject the work of Weismann's already quoted, or Moseley's abstract in Nature, xxix. 1883-4.
The ectoderm on the sides of the blastostyle in the Campanulariae, appears to be frequently multilaminate, and to invest the gonophores more or less loosely. The loose layers are termed gubernaculum by Allman. In Eucopella the medu-soid acquires a chitinoid capsule secreted by the ectoderm filling the cavity of its peculiar blastostyle1. The ova of the Tubularian Wrightia (Atractylis) arenosa undergo development in a gelatinous mass secreted by the walls of the sporosac. In many Campanularians, e.g. species of Sertularia, Calycella, a gelatinoid laminate receptacle, or acrocyst, is formed by the gonophore (? the loose layers of ectoderm) and passes with it out of the gonangium: the ova are fertilised and develope in it, the gonophore undergoing regressive metamorphosis (Allman, Gymnoblastic Hydroids, p. 48; Weismann, op. cit. p. 170). The Sertularian genus Diphasia has this acrocyst protected by processes of the gonangium containing hollow extensions of the tissues of the blastostyle, and together constituting a marsupium (Allman, op. cit. pp. 50-3; Hincks, British Hydroid Zoophytes, p. 244). The Anthome-dusan Eleutheria has a cavity aboral to the atrium and opening into the bell by six interradial canals.
The genital products are formed in the walls of the cavity, which is also a brood-pouch (Hartlaub). So too in Pteronema and Clavatella 1.
1 In Campanopsis ( = Octorchis) the Medusa bud is protected by a layer of ectoderm cells, not by a perisarcal layer, as is usually the case in Tubularians: see Claus, Arb. Zool. Inst. Wien, iv. p. 94 (p. 6 of paper).
Brooks has shown in the planulae of the Anthomedusan Turritopsis and the Leptomedusan Euttma, the presence of an ectodermal invagination, which is afterwards evaginated and exudes a cement attaching the planula to some foreign body (Mem. Boston Soc. Nat. Hist. iii. p. 402-3).
A typical or monoprionid Graptolite, belonging to the section Grapto/oidea, consists of a hollow chitinoid coenosarcal tube or perisarc, which bears along one of its aspects a row of tubular offsets, the cells or thecae. The tube is usually linear, rarely leaf-like; straight or curved; simple or branched; its walls composed of three (?) layers. A solid or hollow chitinoid rod (=virgula) is imbedded in its walls on the aspect opposite to the thecae. The proximal end of the coenosarc is connected to a pointed dagger-like germ or sicula, from which it appears to originate, the pointed end being imbedded in the coenosarc in some families, the broad in others. Two such organisms may be united back to back by their virgular aspects, as in the diprionid Graptolites, or even four, as in the Phyllograptidae. The virgula often projects in these compound forms, more rarely in the monoprionid, far beyond the distal end of the coenosarc; and it may project slightly, as the radicle, at its proximal or sicular end. Specimens of diprionid Graptolites have been found bearing large oval or triangular capsules, supposed to be gonangia.