748 THE ANIMAL KINGDOM. 



invaginated as a stomodaeum in the Acraspeda, and to lay too much stress on the 

 four taeniolae and stomach pouches. The strong general anatomical resemblances 

 between the Acrasped and Craspedote Medusae outweigh any such considerations. 

 See his Abhandlungen zur Entwickelungsgeschichte der Thiere, pt. iv. 1886. 



Metagenesis, or Alternation of Generations in the Hydrozoa, has therefore pro- 

 bably arisen from the larva acquiring the power of forming a colony by gemmation, 

 certain of its progeny only attaining a sexual development. It has itself become at 

 the same time specialised in structure. 



The genus Hydra is probably a direct descendant of the primitive Hydrozoon. 

 Its peculiarities point in this direction. The ambulatory Medusae Clavatella and 

 Eleutheria have been supposed to be intermediate forms between a hydranth and a 

 Medusa, but an entocodon is present in the development of the latter by budding 

 from the Medusa. 



For the remarkable form known as Tetraplatia s. Tetrapteron volitans, see 

 Glaus, A. M. A. xv. 1872 ; Viguier, C. R. 100, 1885. 



Toleration of freshwater by strictly marine forms, e. g. Eucope, Obelia, Sarsia, 

 Turritopsis, Aurelia and its Scyphostoma, Q. J. M. xx. 1880, p. 483-4. Crambessa 

 Tagi is estuarine. Cf. Romanes on the physiology of the freshwater Medusa, 

 Nature, xxii. p. 179. 



Origin of Medusae and the significance of Metagenesis, W. K. Brooks, Mem. 

 Boston Soc. Nat. Hist. iii. pt. 12, 1886. 



Terms used in describing Medusae. Haeckel, Deep-sea Medusae, Challenger 

 Reports, iv. 1882, pp. v-cv, pp. 143-154. 



Fossil Medusae. List in Haeckel's System, Dk. Med. Natw. Ges. Jena, i. 1879, 

 p. 646. 



Phosphorescence, Mclntosh, Nature, xxxii. p. 477 ; Panceri, Atti Accad. 

 Napoli, vii. 1875-7; Allman, Gymnoblastic Hydroids, p. 145; cf. Nature, xxx, 

 Verrill, p. 281, Meldola, p. 289. 



SUB-CLASS I. CRASPEDOTA. 



(Cryptocarpa, Gymnophthahnata, Hydromedusae). 



Hydroid form, either a free and temporary larval stage, or permanent, 

 and then either free or fixed, solitary or colonial either temporarily or perma- 

 nently. It may or may not be tentaculate ; its tentacles rarely hollow, usually 

 solid ; its mouth prominent and gastric cavity simple. It is sometimes poly- 

 morphic. The skeleton is either a chitinoid perisarc, or more rarely a calca- 

 reous coenosteum. Asexual reproduction takes place very rarely by fission, 

 generally by gemmation ; and in the permanent hydroid forms some only of 

 the buds become sexiial zooids. 



The Medusa has a tubular mamibrium, the margin of the bell even or 

 lobed, and provided with an inturned velum, a double nerve ring (inner and 

 outer), and sensory organs either ocelli or auditory organs ; the latter either 

 tentaculocysts or ectodermic otocysts. It may become sessile and degenerate to 

 a greater or less extent. The sexes are separate ; the sexual cells typically of 



