THITIEOID IN TURTLES 281 



Cyclostomata and the hypobranchial groove of Amphioxus, on 

 the one hand, and between the latter and the ventral furrow of the 

 branchial basket in the Tunicata on the other. This homology is 

 based on the following considerations : 



I. The thyreoid of Petromyzon in the Ainmocoetes stage is a 

 sort of muciparous gland, which, if it in some respects differs 

 from the endostyle of the Tunicata, nevertheless presents such 

 points of resemblance to it that the two formations may be 

 regarded as homogeneous (Dohrn, '80). 



II. During the development this gland is transformed into 

 an organ corresponding to the th}Teoid of other vertebrates. 



In the Elasmobranchii the thyreoid is a single medial organ 

 sometimes spherical, sometimes cyhndrical, sometimes triangular 

 or pear-shaped; its volume and weight differ in different animals; 

 sometimes (as in ScylUum catulus) it is cranially located at the 

 angle of bifurcation of the branchial artery; sometimes it is 

 located in the vicinity of the tongue, between the coracohyoideal 

 and coracomandibular muscles (as in Acanthias \^lgaris and in 

 Mustelus lae\ds) ; sometimes it is immediately under the skin 

 (as in Squatina angelus) . 



Within the thyreoid of the Elasmobranchii there exist, accord- 

 ing to Thompson ('10), masses of small solid cells, partly epithe- 

 lial, partly adenoid, which have been thought to indicate a 

 homology with the parathyreoids and the thjanus. 



In the Teleostei, according to ]\Iaurer ('85), the thyreoid is a 

 single organ until, at a certain age, it divides into an accumula- 

 tion of folUcles which surround the branches of the branchial 

 artery on every side. In Amiurus, according to Thomson, the 

 thyreoid consists of a number of vesicles scattered here and 

 there, enclosed in the matrix of the connective tissue; the cells 

 which line the vesicles are cylindrical, very low, and in some cases 

 almost flat. 



In the Urodela the thyreoid is a double organ, yellowdsh in 

 color, spherical or egg-shaped; its largest diameter is less than 

 1 mm.; it is sUghtly flattened, and very superficially located, 

 between the mylohyoid and sternothyreoid muscles, in the 

 immediate vicinity of the jugular vein. 



