CALYPTOBLASTEA LEPTOMEDUSAE 



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 highly 

 specialised organ as the float of Pelagohydra. 



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- 

 gelopsis 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 

 ( rymnoblastea, but is continued into a trumpet-shaped or tubular 

 cup or collar called the " hydrotheca," 

 that usually affords an efficient pro- 

 tection for the zooicls when retracted. 

 No 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 grows 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 Fig. 135. Part of a hydrocladium 

 , *\ . J , . . . of a dried specimen of Plumu- 



ol the colony. In the branching 



Gb, 



laria profunda. Gt, Gono- 

 theca ; He, the stem of the 

 hydrocladium with joints (j) ; 

 Ht, a single hydrotheca ; -V, 

 nematophores. Greatly en- 

 larged. (After Nutting. ) 



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 that have given rise 

 to the branches by gemmation. Such a stem is said to be 

 monosiphonic. 



