("monosiphonic") or several united in a common perisarc ("polysiphonic"). An important variation is seen, in the form of the hydrotheca itself, which may come off from the main stem by a stalk, as in _Obelia_, or may be sessile, without a stalk, as in _Sertularia_. [Illustration: After Allman, _Gymnoblastic Hydroids_, by permission of the council of the Ray Society. FIG. 56.--Diagrams to show the mode of formation of the Hydrotheca and Gonotheca in Calyptoblastic Hydroids. A-D are stages common to both; from D arises the hydrotheca (E) or the gonotheca (F); th, theca; st, stomach; t, tentacles; m, mouth; mb, medusa-buds.] In many Calyptoblastea there occur also reduced defensive polyps or dactylozoids, which in this sub-order have received the special name of _sarcostyles_. Such are the "snake-like zoids" of _Ophiodes_ and other genera, and as such are generally interpreted the "machopolyps" of the _Plumularidea_. These organs are supported by cuplike structures of the perisarc, termed nematophores, regarded as modified hydrothecae supporting the specialized polyp-individuals. They are specially characteristic of the family _Plumularidae_. The medusa-buds, as already stated, are always produced from blastostyles, reduced non-nutritive polyps without mouth or tentacles. An apparent, but not real, exception is _Halecium halecinum_, in which the blastostyle is produced from the side of a nutritive polyp, and both are enclosed in a common theca without a partition between them (Allman [1] p. 50, fig. 24). The gonotheca is formed in its early stage in the same way as the hydrotheca, but the remains of the hydranth persists as an operculum closing the capsule, to be withdrawn when the medusae or genital products are set free (fig. 56). The blastostyles, gonophores and gonothecae furnish a series of variations which can best be considered as so many stages of evolution. Stage 1, seen in _Obelia_. Numerous medusae are budded successively within the gonotheca and set free; they swim off and mature in the open sea (Allman [1], p. 48, figs. 18, 19). Stage 2, seen in _Gonothyraea_. Medusae, so-called "meconidia," are budded but not liberated; each in turn, when it reaches sexual maturity, is protruded from the gonotheca by elongation of the stalk, and sets free the embryos, after which it withers and is replaced by another (Allman [1], p. 57, fig. 28).