768 THE ANIMAL KINGDOM. 



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. {Anisicold) 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, 

 &c. p. 147), as it is in Gonothyrea (p. 133). The young cells lying in the endoderm 

 may be immigrant ectoderm cells. 



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), particu- 

 larly 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, &c. ; 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 blastostyle 1 . 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 



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). 



