25(3 THE DUCTLESS GLANDS 



ductless thyroid, and persists throughout life. The endostyle 

 in Amphioxus and Ammoccetes appears to be a gland which 

 secretes a slime of service in the collection and possibly in the 

 digestion of the food. 



2. Simon found the thyroid in all classes of Fishes. In Elas- 

 mobranchs the thyroid is a moderately large compact organ 

 situated at the anterior end of the ventral aorta, in front of the 

 bifurcation of the branchial artery. The gland is yellowish- 

 white in colour, and the anterior extremity is elongated and 

 prolonged to a point in some species. In other species the 

 thyroid is nearer the end of the tongue, situated on the coraco- 

 hyoid muscles, immediately under the coraco-mandibular, and 

 may be spheroidal, flat, or irregular in shape. Groups of 

 follicles are sometimes detached, forming accessory thyroids. 



There is a striking uniformity in the general microscopic 

 appearance of the thyroid gland throughout all vertebrates 

 above the Cyclostomata, and the thyroid of the Elasmobranch 

 fishes would be immediately recognized as such by any student 

 who had once seen under the microscope a preparation made 

 from the mammalian thyroid. Such differences as are found 

 throughout vertebrates, in regard to the amount of inter- 

 vesicular connective tissue, intervesicular epithelial tissue, size 

 of the vesicles, and so on, are equally to be recognized among 

 the glands of Elasmobranchs themselves. 



Perhaps the most interesting of the species examined is 

 Scyllium canicula. In this fish the thyroid contains not only 

 the usual vesicles, but also several bodies composed of two 

 kinds of cell lymphoid and epithelial. Since parathyroids 

 have not been described by the embryologist, these bodies 

 are probably nodules of thymus. 



Throughout Elasmobranchs the shape of the cells lining the 

 vesicles varies extremely in different species. Thus, in some 

 it is of a tall columnar, and in others of a low cubical, form. 

 The colloid contents appear to be of the same character as in 

 other vertebrates. 



3. Among Teleostean fishes McKenzie gave an account of 

 the anatomy of the thyroid in Amiurus. The gland is placed 

 beneath the copulae of the branchial arches and surrounds the 

 anterior end of the branchial artery. It is an unpaired structure 

 extending in the median line from the origin of the vessels at 

 the first pair of gill arches to a short distance behind the origii 



