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. Margelopsis 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 efficient protection for the zooids when retracted. No solitary Calyptoblastea have been discovered. In the simpler forms the colony consists of a creeping hydrorhiza, 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.
Fig. 135.—Part of a hydrocladium of a dried specimen of Plumularia profunda. Gt, Gonotheca; Hc, the stem of the hydrocladium with joints (j); Ht, a single hydrotheca; N, nematophores. Greatly enlarged. (After Nutting.)
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 composed 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.
In some of the more complicated colonies, however, the stem is composed of several tubes, which may or may not be surrounded by a common sheath of ectoderm and perisarc, as they are in Ceratella among the Gymnoblastea. Such stems are said to be "polysiphonic" or "fascicled." The polysiphonic stem may arise in more than one way, and in some cases it is not quite clear in what manner it has arisen.[[314]]
In many colonies the zooids are only borne by the terminal monosiphonic branches, which receive the special name "hydrocladia." The gonophores of the Calyptoblastea are usually borne by rudimentary zooids, devoid of mouth and tentacles (the "blastostyles"), protected by a specially dilated cup of perisarc known as the "gonotheca" or "gonangium." The shape and size of the gonothecae vary a good deal in the order. They may be simply oval in shape, or globular (Schizotricha dichotoma), or greatly elongated, with the distal ends produced into slender necks (Plumularia setacea). They are spinulose in P. echinulata, and annulated in P. halecioides, Clytia, etc.
In some genera there are special modifications of the branches and hydrocladia, for the protection of the gonothecae. The name "Phylactocarp" is used to designate structures that are obviously intended to serve this purpose. The phylactocarp of the genera Aglaophenia and Thecocarpus is the largest and most remarkable of this group of structures, and has received the special name "corbula." The corbula consists of an axial stem or rachis, and of a number of corbula-leaves arising alternately from the rachis, bending upwards and then inwards to meet those of the other side above, the whole forming a pod-shaped receptacle. The gonangia are borne at the base of each of the corbula-leaves. There is some difference of opinion as to the homologies of the parts of the corbula, but the rachis seems to be that of a modified hydrocladium, as it usually bears at its base one or more hydrothecae of the normal type. The corbula-leaves are usually described as modified nematophores (vide infra), but according to Nutting[[315]] there is no more reason to regard them as modified nematophores than as modified hydrothecae, and he regards them as "simply the modification of a structure originally intended to protect an indefinite person, an individual that may become either a sarcostyle[[316]] or a hydranth."
The other forms of phylactocarps are modified branches as in Lytocarpus, and those which are morphologically appendages to branches as in Cladocarpus, Aglaophenopsis, and Streptocaulus.