HYDROZOA CRASPEDOTA. 777 



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. 



(iii) Campanulariae s. Calyptoblastea. Hydranth furnished with a single 

 circle of filiform tentacles with a solid axis ; perisarc, as a rule, forming hydrothecae 

 and gonothecae, 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, &c. ; Plumularidae, thecae sessile, uni- 

 serial, with machopolypes, e.g. Plumularia, Antennularia, Aglaophenia, &c. ; Sertu- 

 laridae, thecae sessile, biserial, e.g. Sertularia, Diphasia, Halecium, &c. *. 



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, Physo- 

 phoridae, Agalmidae, Apolemiadae, Rhizophysidae. 



(ii) Physaliidae : pneumatophore large, fusiform, with an aperture ; no necto- 

 calyces nor hydrophyllia. Female zooid a Medusa (?). Physalia. 



(iii) Discoideae : coenosarc a disc ; air-vesicle composed of concentric cham- 

 bers, more or fewer of which open externally ; a central polypite surrounded by a 

 zone of blastostyles, the latter by a zone of tentacles 2 . 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 necto- 

 calyx 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. 



I. Trachy medusae, Haeckel, System der Medusen, Jena, 1879, pp. 234-359 ; 

 Id. Deep-sea Medusae, Challenger Reports, iv. 1882, pp. 9-48; anatomy of various 

 species, see in O. and R. Hertwig, Das Nervensystem und die Sinnesorgane der 

 Medusen, Leipzig, 1878; lid. Organismus der Medusen, Jena, 1878; Nervous 

 system, &c., of Carmarina, Eimer, Die Medusen physiologisch, &c., auf ihr Nerven- 



1 For another classification cf. von Lendenfeld, Z. A. vii. 1884. 



2 Beneath the central gastrozooid of the Discoideae lies a mass of tissue often spoken of as 

 ' liver.' Its structure has been worked out by Bedot, who terms it ' central organ.' It consists 

 essentially of a mass of cnidoblasts and tubes. The latter are connected to the central zooid and the 

 canals of the coenosarc, and form a dense layer beneath the pneumatophore, connected to a second 

 but less dense superficial layer. The cells of the first layer contain coloured corpuscles in Velella,. 

 granules and crystals of guanine in Porpita. The blastostyles and tentacles (dactylozooids) are con- 

 nected to the canal system. 



