The Craspedota are classified as follows

I. Order Trachy Medusae, 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.

(i) 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, Petasidae, Trachynemidae, Aglauridae, Geryonidae.

II. Order Hydroidea

Hydranth small as a rule; sexual only in Hydra; sometimes 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 arrangement; 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 belllike, ocellate, belonging to the Anthomedusae, see ante, p. 760. Hydra, Clava, Cordylophora, Coryne, Syncoryne, Eudendrium, Tubularia, Corymorpha, Hydractinia, Podocoryne, etc.

(II) Hydrocorallina

Hydranth polymorphic, either a gastrozooid (pp. 758-9) or dactylozooid (p. 758); colonial; skeleton a calcareous coenosteum with gastro-and dactylo-pores (p. 756). (1) Milleporidae: coenosteum arborescent or encrusting 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 sometimes confined to the margins or to one surface of the colony. Colonies of separate sexes; sexual zooid, a sporosac, developed on the coenosarcal tubes and contained in a special cavity or ampulla of the coenosteum, several sporosacs in one ampulla in the male, a single one in the female. Many genera, e. g. Pliobothrus, Stylaster, Cryptohelia.

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

(III) Campanulariae S. Calyptoblastea

Hydranth furnished with a single circle of filiform tentacles with a solid axis; perisarc, as a rule, forming hydrothecae and gonoihecae, structures absent in Campanopsis (=Octorchis), Eutima, Zygodactyla (=Aequorea); a specialised blastostyle in all cases where gonothecae are developed; Medusa disc-like, either ocellate or vesiculate, i.e. provided with otocysts, belonging to the Leptomedusae (pp. 760-1, ante). Campanularidae with pedunculate thecae, e.g. Campanularia, Clytia, Obelia, Gonothyrea, etc.; Plumularidae, thecae sessile, uni-serial, with machopolypes, e.g. Plumularia, Aniennularia, Aglaophenia, etc.; Sertu-laridae, thecae sessile, biserial, e. g. Sertularia, Diphasia, Halecium, etc.l.

III. Order Siphonophora

Pelagic, colonial; zooids polymorphic.

(i) Physophoridae: coenosarc, either short and sac-like or long and tubular, spirally twisted; a flask-shaped pneumatophore; nectocalyces as a rule present, and distal to the pneumatophore, disposed in two or several rows. Hydrophyllia and hydrocysts nearly always present, alternating regularly with the polypites and sexual zooids. Female zooid produces but one ovum. Athorybiadae, Physophoridae, Agalmidae, Apolemiadae, Rhizophysidae.

(ii) Physaliidae: pneumatophore large, fusiform, with an aperture; no nectocalyces nor hydrophyllia. Female zooid a Medusa (?). Physalia.

(iii) Discoideae: coenosarc a disc; air-vesicle composed of concentric chambers, more or fewer of which open externally; a central polypite surrounded by a zone of blastostyles, the latter by a zone of tentacles2. Sexual zooid a free medusa. Porpita; a vertical crest, Velella, young known as Rataria.

(iv) Calycophoridae: a single nectocalyx (Monophyidae), or two (Diphyiidae), or a double series (Hippopodiidae). No hydrocysts. Zooids in groups on an elongated tubular coenosarc, retractile into a groove or canal of the distal nectocalyx except in Hippopodiidae. The groups are detached as Diphyozooids in some species of Diphyes, in Abyla, and Monophyidae. Female zooid produces many ova. Hippopodiidae, see note, p. 773, ante; Diphyidae; Monophyidae, see pp. 774-5, ante.