with a form such as Corymorpha, which also is not fixed but only rooted in the mud. The medusae, on the other hand, have the tentacles in four tufts of (in the buds) five each, and thus resemble the medusae of the family Margelidae. See A. Dendy .
Perigonimus.—This common British hydroid belongs by its characters to the family Bougainvillidae; it produces, however, a medusa of the genus Tiara (fig. 52), referable to the family Clavidae; a fact sufficient to indicate the tentative character of even the most modern classifications of this order.
Sub-order II. Hydroidea Calyptoblastea (Leptomedusae).—Trophosome with polyps always differentiated into nutritive and reproductive individuals (blastostyles) enclosed in hydrothecae and gonothecae respectively; with sympodial type of budding. Gonosome with free medusae or gonophores; the medusae typically with otocysts, sometimes with cordyli or ocelli (figs. 54, 55).
Fig. 55.—View of the Oral Surface of one of the Leptomedusae (Irene pellucida, Haeckel), to show the numerous tentacles and the otocysts.
The calyptoblastic polyp of the nutritive type is very uniform in character, its tendency to variation being limited, as it were, by the enclosing hydrotheca. The hydranth almost always has a single circlet of tentacles, like the Bougainvillea-type, in the preceding sub-order; an exception is the curious genus Clathrozoon, in which the hydranth has a single tentacle. The characteristic hydrotheca is formed by the bud at an early stage (fig. 56); when complete it is an open cup, in which the hydranth develops and can be protruded from the opening for the capture of food, or is withdrawn into it for protection. Solitary polyps are unknown in this sub-order; the colony may be creeping or arborescent in form; if the latter, the budding of the polyps, as already stated, is of the sympodial type, and either biserial, forming stems capable of further branching, or uniserial, forming pinnules not capable of further branching. In the biserial type the polyps on the two sides of the stem have primitively an alternating, zigzag arrangement; but, by a process of differential growth, quickened in the 1st, 3rd, 5th, &c., members of the stem, and retarded in the 2nd, 4th, 6th, &c., members, the polyps may assume secondarily positions opposite to one another on the two sides of the stem. Other variations in the mode of growth or budding bring about further differences in the building up of the colony, which are not in all cases properly understood and cannot be described in detail here. The stem may contain a single coenosarcal tube (“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.
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  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 , 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 , p. 57, fig. 28).
Stage 3, seen in Sertularia.—The gonophores are reduced in varying degree, it may be to sporosacs; they are budded successively from the blastostyle, and each in turn, when ripe, protrudes the spadix through the gonotheca (fig. 57, A, B). The spadix forms a gelatinous cyst, the so-called acrocyst (ac), external to the gonotheca (gth), enclosing and protecting the embryos. Then the spadix withers, leaving the embryos in the acrocyst, which may be further protected by a so-called marsupium, a structure formed by tentacle-like processes growing out from the blastostyle to enclose the acrocyst, each such process being covered by perisarc like a glove-finger secreted by it (fig. 57, C). (Allman , pp. 50, 51, figs. 21-24; Weismann , p. 170, pl. ix., figs. 7, 8.)