While in many respects more primitive than in Elasmobranchs the branchial system of the Cyclostomata presents certain special and peculiar features. The branchial clefts assume the form of oval, antero-posteriorly flattened pouches or sacs, varying, however, in number, and in their mode of communicating with the exterior, in different genera. In the Lamprey (_Petromyzon_) there are seven pairs of obliquely-disposed gill-sacs opening externally by small rounded orifices, and by similar apertures, not directly into the pharynx, but into a branchial canal (Fig. 162, _r.t_), which underlies the oesophagus, and, while ending blindly behind the last pair of sacs, communicating in {280}front with the oral cavity.[279] The first of the series of gill-sacs corresponds to the hyo-branchial or hyoidean cleft of Elasmobranchs and other Fishes. Spiracles are absent in the adult, but in the embryo are represented by pouch-like outgrowths of the hypoblast of the oral cavity, which subsequently undergo singular changes.[280] Thus, the outgrowths become converted into the lateral halves of a complete ciliated circum-oral groove, which is retained even in the Ammocoetes stage, and recalls the ciliated peripharyngeal ring of Ascidians. Another archaic feature is also to be noted in the continuity of the groove with a ciliated mid-dorsal pharyngeal ridge, which has been compared to the "dorsal lamina" of Ascidians, and to the equally characteristic hyperbranchial groove of _Amphioxus_.[281] Ventrally also, the lateral halves of the groove unite to form a single groove, which, after receiving the median aperture of the thyroid rudiment,[282] is continued backwards in the mid-ventral line of the pharyngeal wall as far as the last branchial arch. No trace of these ciliated structures is, however, to be met with in the adult. [Illustration: FIG. 162.—_Petromyzon marinus_. Transverse section through the branchial region (semi-diagrammatic). _br.m_, Branchial membrane; _d.ao_, dorsal aorta; _d.c_, dorsal cartilage of the branchial basket; _d.m_, dorsal muscles; _e.a_, external aperture of a gill-sac; _f.t_, fibrous tissue enclosing neural canal; _h_, _i_, lateral longitudinal cartilages of the branchial basket; _i.a_, internal aperture of a gill-sac; _i.ju_, inferior jugular vein; _ju_, jugular vein (anterior cardinal); _my_, spinal cord; _nc_, notochord; _n.ca_, neural canal; _n.p_, neural process; _oes_, oesophagus; _p.br_, peri-branchial lymph sinus; _r.m.t_, retractor muscle of the tongue; _r.t_, respiratory tube or branchial canal; _s_, circum-oesophageal lymph sinus; _v.ao_, ventral aorta; _v.c_, ventral cartilage of branchial basket; _v.m_, ventral muscles. (From T. J. Parker.)]