766 THE ANIMAL KINGDOM. 



Silurian, and by a few other Silurian and Devonian forms. Serttdarella 

 polyzonias occurs in Miocene strata in Ayrshire : a Milleporan (Porosphaerd) 

 in the Chalk : Stylaster in Miocene strata. 



The freshwater Polypodium hydriforme is a parasite in its first observed stage 

 in the ova of the Sterlet (Acipenser ruthenus). It is then a cylindrical tube or stolon 

 which gives origin to 16 buds, each of which divides once, forming 32 in all. Each 

 bud acquires 24 hollow tentacles. The stolon breaks up when transferred to fresh 

 water. The free Polypodium is 2 mm. long and 4^ mm. wide. It divides by longi- 

 tudinal fission into 2 parts with 12 tentacles apiece, and these in turn into 2 with 6 

 tentacles apiece. All these forms grow, acquire 24 tentacles, and then divide 

 again. Polypodium moves from place to place by means of its tentacles, on which it 

 stands like stilts. Ussow, A. N. H. (5), xviii. 1886. 



The hydranth is frequently detached in Tubularta, and may live some time in 

 this condition, a new hydranth developing as a bud in its place. The two Tubularian 

 genera Nemopsis and Acaulis are considered by Allman to be similarly detached 

 hydranths, and the Hydroid of the former has been discovered by Brooks (Studies 

 Biol. Lab. John's Hopkins Univ. ii. p. 468). A complete absence of hydranths 

 during winter has been observed in Eudendrium, Cordylophora and Hydractinia, but 

 the coenosarc persists and develops buds again at a more favourable season 1 . So 

 too, when the destruction of Campanularidan colonies by parasitic Protophytes 

 takes place, the hydrocope of certain hydranths may grow. suddenly for an inch or 

 so, and then develops a new colony at a higher level (von Lendenfeld, Z. A. vi. p. 

 42). The power of repairing injuries is very great, and propagation of the indi- 

 vidual or colony by artificial section is possible, as in Hydra and Cordylophora. 



A few points in the histology of the hydroid colony may be noted. The 

 ectoderm forms an unbroken layer on the surface of the hydrophyton in the 

 Hydractinidae and Hydrocorallina. Ectodermal ganglion cells occur on the arms, 

 chiefly towards their bases (cf. ante, p. 329); also, but more sparingly as a rule, on 

 the body, occasionally aggregated at its base, and even present in numbers in the 

 coenosarc (Eudendrium ramosuni). Sense cells are present, and the sense hairs 

 may have great length as in the palpocils of Syncoryne. The ectodermal muscle 

 cells, epithelial or sub-epithelial, are always disposed longitudinally. In the 

 coenosarc the ectoderm is usually unilaminar, but it may be multilaminar as in 

 Eudendrium ramosum, and its cnidoblasts are few and devoid of cnidocils. It is 

 usually said not to contain muscle-cells, but they have been detected in Campanu- 

 laria and Plumularia, where they cause slow rhythmic contractions of the cavity of 

 the tube, and they are probably present in other instances. It secretes the perisarc, 

 and is either in contact with it at all points or only here and there by cells with a 

 striated structure ; cf. von Lendenfeld, Z. W. Z. xxxviii. p. 534, and for the chitino- 

 genous cells, ibid. p. 520. Where perisarc is to be absorbed the ectodermal cells 

 enlarge ; see op. cit. p. 542, and Weismann, Entstehung der Sexualzellen, &c. pp. 

 178 and 182. The other constituents of the ectoderm are epithelial cells, inter- 

 stitial cells, and gland cells. As to the endoderm, ganglion and sense cells have 

 been found in the hypostome of Eucopella and of other Campanularians (von 



1 This phenomenon is perhaps not uncommon : see Hincks, op. cit. p. xliv. On Eudendrium 

 see Weismann, Entstehung, &c. p. 102 ; and on Tubularia, Allman, op. cit. p. 403. 



