. The elasmobranch fishes . 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). Wh
. The elasmobranch fishes . 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; Squatina) 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.
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