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. [Illustration: 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. {276}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.