770" THE ANIMAL KINGDOM. 



gives rise to more than one zooid is simply an instance of precocious gemmation, to 

 be paralleled by the fission of the embryo of Lumbricus trapezoides, and possibly of 

 the Acrasped Chrysaora. It may be noted also that the Hydroid planula does not 

 always form the hydranth directly; e.g. mEutima it attaches itself and produces a 

 hydranth by gemmation, itself becoming a hydrorhiza l . 



The Craspedota are classified as follows 



I. Order Trachymedusae Trachylinae (Haeckel). Tentacles solid, sometimes 

 replaced in part in the adult by hollow ; tentaculocysts or auditory organs with 

 endodermal axis, containing one or more otoliths. Development by metamorphosis 

 from free hydroid larvae. 



(1) Narcomedusae : tentaculocysts always free; tentacles at some distance 

 from margin of bell to which they are connected by peronia ; margin of the bell 

 lobed ; radial pouches to the stomach sometimes present, sometimes absent ; festoon 

 canals generally present; genital organs on the subumbrellar aspect of stomach, 

 extending beneath radial pouches or restricted to the latter. Four families, Cunan- 

 thidae, Peganthidae, Aeginidae, Solmaridae. 



(2) Trachomedusae : tentaculocysts either free or inclosed in capsules, which 

 may be sunk in the mesoglaea ; radial canals (4, 6, 8) and a circumferential canal ; 

 caecal centripetal canals sometimes present; genital organs on the subumbrellar 

 aspect of the radial canals. Four families, Petastdae, Trachynemidae, Aglauridae, 

 Geryonidae. 



II. Order Hydroidea. Hydranth small as a rule ; sexual only in Hydra ; some- 

 times non-colonial, but usually giving origin by gemmation to a plant-like colony ; 

 often polymorphic. An exo-skeletal perisarc usually investing the coenosarc, chiti- 

 noid, sometimes calcareous forming a coenosteum. Sexual zooid or Medusa cras- 

 pedote with ocelli or entirely ectodermic otocysts, produced by gemmation from 

 the coenosarc or a hydranth ; frequently more or less degenerate, see p. 762, and 

 p. 768. 



(i) Tubulariae s. Gymnoblastea. Hydranth of very variable appearance, non- 

 tentaculate in Micro- and Proto-hydra ; its tentacles varying in character and arrange- 

 ment; rarely specialised as a blastostyle; sometimes non-colonial (ante, p. 755), 

 and in Hydra locomotive ; usually colonial ; coenosarc of colony invested by a 

 perisarc which never forms hydro- or gono-thecae but generally extends on to the 

 hydrocephalis, and in Bimeria even invests the bases of the tentacles ; Medusa bell- 

 like, ocellate, belonging to the Anthomedusae, see ante, p. 760. Hydra , C/ava, 

 Cordylophora, Coryne, Syncoryne, Eudendrium, Tubularia, Corymorpha, Hydractinia^ 

 Podocoryne, &c. 



(ii) Hydrocorallina. Hydranth polymorphic, either a gastrozooid (pp. 758-9) 

 or dactyl ozooid (p. 758) ; colonial ; skeleton a calcareous coenosteum with gastro- 

 and dactylo-pores (p. 756). (i) Milleporidae : coenosteum arborescent or encrust- 

 ing composed of a superficial living and a number of deep dead layers; pores 

 crossed by tabulae, either scattered or in systems with dactylopores grouped round 

 a central gastropore ; sexual products developed in capsules of the coenosarcal 

 canals ; Millepora. (2) Stylasteridae : coenosteum arborescent ; either entirely 

 living or with deeper dead layers ; pores either scattered or grouped in systems 



1 Brooks, op. cit. ante, p. 403 ; and cf. pp. 764-5, ante. 



