This group is clearly distinguished from the second sub-order, the Hemimyaria, by the condition of the muscle-bands and of the branchial sac, and by the life-history. The muscle-bands are complete rings (except in _Anchinia_), while in the Hemimyaria they are always more or less incomplete. The branchial sac in the Cyclomyaria is a distinct cavity, and communicates with the peribranchial cavity only by small slits or stigmata. The life-history is also very characteristic, as the sexual generation in the Cyclomyaria is always polymorphic, while in the Hemimyaria it consists of one form only. [Illustration: FIG. 59.—Sexual generation of _Doliolum tritonis_, Herdman, from left side, × 10. _at_, Atrial aperture; _at.l_, atrial lobes; _at.m_, wall of atrium; _br_, branchial aperture; _br.l_, branchial lobes; _br.s_, branchial sac; _d.t_, dorsal tubercle; _end_, endostyle; _h_, heart; _i_, intestine; _m_, mantle; _m_^1-_m_^8, circular muscle-bands; _n_, nerve; _n.g_, nerve-ganglion; _ov_, ovary; _p.br_, peribranchial cavity; _p.p_, peripharyngeal bands; _sg_, stigmata; _s.gl_, neural gland; _s.o_, sense-organ; _st_, stomach; _t_, test; _tes_, testis; _z_, prebranchial zone. (After Herdman.)] STRUCTURE OF DOLIOLUM.—The single family DOLIOLIDAE includes three genera, _Doliolum_, Quoy and Gaimard, _Dolchinia_, Korotneff, and _Anchinia_, Eschscholtz. _Doliolum_, of which about a dozen species are known, from various seas, has a cask-shaped body (Fig. 59), usually from 1 to 2 cm. in length. The terminal branchial and atrial apertures are lobed, and the lobes are provided with sense-organs. The test is a thin but tough transparent layer, and contains no "test" cells. It is merely a cuticle covering the surface of the squamous ectoderm. The body-wall has eight or nine circular muscle-bands surrounding the body. The most anterior and posterior of these form the branchial and atrial sphincters. The wide branchial and atrial apertures lead respectively into branchial and peribranchial cavities separated by the posterior and postero-lateral walls of the branchial sac which are pierced by a considerable number of {97}small stigmata; consequently there is a free passage for the water through the body along its long axis, and the animal swims by contracting its ring-like muscle-bands so as to force out the contained water posteriorly. When stigmata are found on the lateral walls of the branchial sac (see Fig. 59) there are corresponding anteriorly directed diverticula of the peribranchial cavity. There is a distinct endostyle on the ventral edge of the branchial sac and a peripharyngeal band surrounding its anterior end, but there is no representative of the dorsal lamina along its dorsal edge; and there are neither branchial nor atrial tentacles. The oesophagus commences rather on the ventral edge of the posterior end of the branchial sac, and runs backwards to open into the stomach, which is