764 T H E ANIMAL KINGDOM. 



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 

 Eudendriitm, 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, 

 Ttibularia indivisa, Myriothela, undergo atrophy, and the products to which 

 they give origin serve as food-material to the remainder. A vitelline mem- 

 brane is absent in all ova borne by a Medusa, and as a rule when develop- 

 ment 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, &c. The Medusae Bougainvillea stiperciliaris and Ametrangia 

 hemisphaerica are viviparous *. Segmentation is total, rarely very unequal 

 as in Clava squamata, where the hypoblast is represented by two cells for 

 a long time. 



The embryo is usually from the first a solid mass of cells, the outer 

 layer of which is ciliated and differentiated as epiblast : or there is a blasto- 

 coele, as in Hydra and most embryoes produced from the eggs of Medusae, 

 e. g. Eutima, Obelia, Eiicope, Aequorea 2 . In the first case the internal cells 

 arrange themselves in a layer : in the second the blastocoele is, sooner or 

 later, filled with cells derived (i) by the growth and fission of the cells of the 

 wall in general (Eucope), and the embryo has then become a planula, or 

 of one pole, the most usual mode, or (2) by migration inwards (Obelia). 

 The gastric cavity is formed as a central slit, or the peripheral endoderm 

 cells devour the central, as in Clytia and Obelia (Metschnikoff), or it is in 

 Eutima a remnant of the blastocoele. The planula leads a free existence, 

 and then fixes itself, either by one pole or by its side. In the first case it 

 lengthens and the mouth and tentacles appear at the free end, or it flattens 

 into a disc, from the centre of which rises a bud, the future hydranth {Clytia, 

 Obelia, Eucope, Eudendrium, Campamdaria, Plumularia, Serttdaria] : in the 

 second, either one end becomes hydrorhiza, the other grows and becomes 

 the hydranth ( Tiara, Eutima), or the hydranth-bud appears laterally ( Tur- 

 ritopsis nutriculd), or it forms a ramified and anastomosing hydrorhiza, 

 from which buds are developed (Turritopsis (Oceania] armata). In Mitro- 

 coma Annae the planula may become either hydrorhiza alone or.hydrorhiza 



1 Cf. Wagner, Wirbellosen des Weissen Meeres, p. 74; Allman, Nature, ix. p. 74. In the 

 last instance the planula is oval and non-ciliated. On the species, which is perhaps fissiparous, see 

 Haeckel, System, p. 636. 



2 According to Metschnikoff, two or three blastulae may fuse in Mitrocoma Annae ; but develop- 

 ment proceeds normally. 



