[Illustration: _Fig. 54._--Diagram showing possible modifications of the persons of a Calyptoblastic Hydromedusa. Letters a to h same as in fig. 49. i, The horny cup or hydrotheca of the hydriform persons; l, medusiform person springing from m, a modified, hydriform person (blastostyle); n, the horny case or gonangium enclosing the blastostyle and its buds. This and the hydrotheca i give origin to the name _Calyptoblastea_. (After Allman.)] _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). [Illustration: FIG. 55.--View of the Oral Surface of one of the _Leptomedusae_ (_Irene pellucida_, Haeckel), to show the numerous tentacles and the otocysts. ge, Genital glands. M, Manubrium. ot, Otocysts. rc, The four radiating canals. Ve, The velum.] 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it is an open cup, in which the hydranth develops and can be protruded from the opening for the capture of food, or is withdrawn into it for protection.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