344 



FISHES 



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 Lepidosiren, and was regarded 

 by him as a salivary gland. 



As in Eeptiles, 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 



Sp.C. 



^"-^ •5°- brc: th 



Fig. 202. — A, a vertical section through a just-hatched larva of Petromyzon. a.% 

 Auditory vesicle ; hrx, branchial cleft ; h, heart ; m, mouth ; yi, 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; 

 t, thymus ; th, 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 



^ Quoted by N. Parker, I.e. 



^ Van Bemmelen, Anat. Anz. iv. 1889, p. 400. ^ W. Miiller, op. cit. 



