344 FISHES CHAP. 
sisting of two lateral lobes connected by a constricted median 
portion, and situated beneath the epithelium of the tongue, 
immediately above the hyoidean symphysis. <A similar structure 
has been described by Bischoff! in Zepidosiren, and was regarded 
by him as a salivary gland. 
As in Reptiles, Birds, and Mammals, paired or accessory 
thyroid bodies (“ supra-pericardial organs ”)” 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 
DP. bro: th. ph. hh 
Fia. 202.—A, a vertical section through a just-hatched larva of Petromyzon. «av, 
Auditory vesicle; 6r.c, branchial cleft; h, heart; m, mouth; 2, 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 develop- 
ment 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 ; 
¢, thymus ; ¢h, median thyroid. (From Hertwig, after de Meuron.) 
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. 
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 develop- 
ment than the view® that the organ is derived from the 
characteristic hypobranchial groove or “endostyle” of Ascidians 
1 Quoted by N. Parker, Z.c. 
? Van Bemmelen, Anat. Anz. iv. 1889, p. 400. 3 W. Miiller, op. cit. 
