CALYPTOBLASTEA LEPTOMEDUSAE 



275 



of them have been shown to produce numerous free-swimming 

 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 liighly 

 specialised organ as the float of Pelagoliydra. 



The affinities of Pelagohydra 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- 

 gelofsis 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 

 G-ymnoblastea, but is continued into a trumpet-shaped or tubular 

 cup or collar called the " hydrotheca," 

 that usually affords an efficient pro- 

 tection for the zooids when retracted. 

 No solitary Calyptoblastea have been 

 discovered. In the simpler forms the 

 colony consists of a creeping hydro- 

 rliiza, from which the zooids arise 

 singly (Clytia Johnstoni), but these 

 zooids may give rise to a lateral bud 

 which grows longer than the parent 

 zooid. 



The larger colonies are usually 

 formed by alternate right and left 

 budding i'rom the last-formed zooid, 

 so that in contrast to the Gymnoblast 

 colony the apical zooid of the stem is 

 the youneest, and not the oldest, zooid pm. l35.-Partof ahydrociadmm 



J ^ ' ^ , , , . of a dried specimen of Plumu- 



of the colony. In the branchmg 

 colonies the axis is frequently com- 

 posed of a single tube of perisarc, 

 which may be lined internally by 

 the ectoderm and endoderm tissues 

 formed by the succession of zooids 

 to the branches by gemmation. Such 

 monosiphonic. 



specimen ( 

 laria profunda. Gt, Gono- 

 theca ; He, the stem of the 

 hydrocladium with joints {]) ; 

 lit, a single hydrotheca ; N, 

 nematophores. Greatly en- 

 larged. (After Nutting.) 



that have 

 a stem is 



given rise 

 said to be 



