CALVPTOBLASTEA LEPTOMEDUSAE 



75 



of them have been shown to produce luuuL'rous five-swimmiiig 

 Medusae by gemmation ; but at present there is no reason to 

 suppose that in these forms there is any extensive modification 

 of the aboral extremity of the zooid to form such a highly 

 specialised organ as the float of Felagohydra. 



The af&nities of Pclagohydra are not clear, as our knowledge 

 of the characters of the Medusa is imperfect ; but according to 

 Dendy it is most closely related to the Corymorphidae. Mar- 

 (jelopsis belongs to the Bougainvilliidae. 



Order IV. Calyptoblastea — Leptomedusae. 



The hydrosome stage is characterised by the perisarc, which 

 not only envelops the stem and branches, as in many of the 

 Gymnoblastea, but is continued into a trumpet-shaped or tubular 

 cup or collar called the " hydrotheca," 

 that usually affords an ef&cient pro- 

 tection for the zooids when retracted. 

 Xo solitary Calyptoblastea have been 

 discovered. In the simpler forms the 

 colony consists of a creeping hydro- 

 rhiza, from which the zooids arise 

 singly {Clytia johnstoni), but these 

 zooids may give rise to a lateral bud 

 which growls longer than the parent 

 zooid. 



The larger colonies are usually 

 formed by alternate right and left 

 budding from the last-formed zooid, 

 so that in contrast to the Gymnoblast 

 colony the apical zooid of the stem is 

 the youngest, and not the oldest, zooid 

 of the colony. In the branching- 

 colonies the axis is frequently com- 

 posed of a single tube of perisarc, 

 wdiich may be lined internally by 

 the ectoderm and endoderm tissues 

 formed by the succession of zooids 

 to the branches by gemmation. Such 

 monosiphonic. 



Fig. 135. — Part of a liydrocladiuiu 

 of a dried specimen of Plumu- 

 luria profunda. Gt, Gono- 

 tlieca ; He, the stem of the 

 hy<lroclailium with joints (j) ; 

 Ht. a single hydrotheca ; N, 

 neniatophores. Greatly en- 

 larged. (After Xnttiug.) 



that have given rise 

 a stem is said to be 



