THE ELASMOBRANCH PISHES 



133 



foi'med directly into lyiii|)li()(*ytos. It is generally believed, however, that the 

 function of the thyniu.s, whatever it may be, is not that of a l,vniphoid organ 

 (Fritsehe, 1910). 

 That the thynuis possesses a tlnct in Heptanvlius, as has been demonstrated 



Fig. 131. Thyroid follicles making up the gland. (From Ferguson.) A. Carcharias Uttortilis. 

 B. Eaia erivacea. 



by Van Wijhe, is an unusually interesting fact, although the significance of 

 the duct has not yet been made out. 



The thyroid- gland in the Elasmobranch fishes is a gelatinous mass of tissue 

 surrounded by a connective tissue capsule. In the sharks it is located in the 

 region behind or below the basihyal cartilage, 

 between the coracomandibular and the coraco- 

 hyoideus muscles. It may be creseentic in «ii'T^^e 

 or it may be more or less irregular {Acanthias) . 

 Where the basihyal cartilage is broad it occupies 

 its ventral side, resting in a depression {Acanfhias, 

 Mustelns) or in a deep groove {Carcharias Uito- 

 ralis) . It occupies the space between the basihyoid 

 and the bifurcation of the ventral aorta. Where 

 the cartilage is narrow as in Raia erinacea the 

 thyroid may lie farther posterior on the terminal 

 bifurcation of the ventral aorta. 



Upon removal of the connective tissue capsule 

 from the thyroid it is seen to be made up of groups 

 of follicles of various forms. They may be small 

 (Carcharias, fig. 131a ; Mustelus; Squat ina) or 

 large (Raia, fig. 131b). A section through an in- 

 dividual follicle shows an outer wall of epithelium 

 enclosing a mass of colloidal substance (Fergu- 

 son, 1911). 



The history of the thyroid in forms lower than the Elasmobranchs is of in- 

 terest. In Amphioxus, the Ascidians, and Ammocoetes, there is present in the 

 floor of the pharynx a median groove, the ciliated endostyle, the walls of which 



2 Thyreoid. 



Fig. 132. Sagittal section 

 through the thyroid gland of 

 CMamydoselachus anguineus. 

 (From Goodey.) 



fl., follicle ; s.d., remnant of 

 stomodeal denticle; t.d., thy- 

 roid duct. 



