As in Reptiles, Birds, and Mammals, paired or accessory thyroid bodies ("supra-pericardial organs")[399] are present in many Fishes, and appear to be similar in structure to the median thyroid. In Elasmobranchs these bodies originate as a pair of outgrowths from the epithelium of the pharynx behind the last pair of branchial arches (Fig. 202, B, _a.th_). Subsequently they become detached from the pharynx, and in the adult are situated on the dorsal side of the pericardium, remote from the median thyroid. [Illustration: FIG. 202.—A, a vertical section through a just-hatched larva of _Petromyzon_. _a.v_, Auditory vesicle; _br.c_, branchial cleft; _h_, heart; _m_, mouth; _n_, notochord; _ol_, olfactory pit; _ph_, pharynx; _sp_, septum or velum between the stomodaeum and the mesenteron; _sp.c_, spinal cord; _th_, thyroid outgrowth from the floor of the pharynx. (From Gegenbaur, after Calberla.) B, diagram illustrating the development of the thyroid, the thymus, and the accessory thyroids, and their relations to the branchial clefts. _a.th_, Accessory thyroids; _g.p_, gill-pouches; _Ph_, pharynx; _t_, thymus; _th_, median thyroid. (From Hertwig, after de Meuron.)] According to Dohrn the median thyroid is to be regarded as the vestige of a gill-cleft which primitively existed between the hyomandibular cartilage and the hyoidean arch. This conclusion seems, however, to be less in harmony with the facts of development than the view[400] that the organ is derived from the characteristic hypobranchial groove or "endostyle" of Ascidians {345}and Amphioxus, which has undergone a change of function from a mucus-conveying groove to a blood-gland. On the other hand, the mode of origin of the paired thyroids certainly favours the suggestion that they represent a posterior pair of vestigial gill-clefts, a view which derives some support from the fact that in _Notidanus_, where additional branchial arches and clefts are present, the paired thyroids are absent.