196 



THE ORIGIN OF VERTEBRATES 



cavity of the portion A or A' is formed by the bending over of the tube 

 or horn, B or B', with the partial absorption of the septum so formed 

 between the tube and its bent-over part. If, then, we uncoil the 

 curled-up part of C, and separate the portion, B, on each side from the 

 chamber, C, we see that the so-called thyroid of Ammoccetes may be 

 represented as in Fig. 83, i.e. it consists of a long, common chamber, C, 



Ps br! 



Th. o .. -v-- : ''' 



Pit, 



•) * 



B 



Fig. 82. — Diagbammatic Repbesentation of the so-called Thyboid Gland op 



Ammoccetes. 



C, central chamber; A, A', anterior extremity; B,B', posterior extremity; Tli.o., 

 thyroid opening into respiratory chamber; Ps. br., Ps. br'., ciliated grooves, 

 Dohrn's pseudo-branchial grooves. 



Fig. 83.— Thyboid Gland as it would appeab if the Centbal Chambeb were 

 Uncueled and the Two Hoens, B, B', sepaeated fbom the Centbal 

 Chambeb. 



which, for reasons apparent afterwards, I will call the palceo-hysteron, 

 which opens, by means of a large orifice, into the respiratory or 

 pharyngeal chamber. The anterior end of this chamber terminates in 

 two tubes, or horns, B, B', the structure of which shows that the median 

 chamber, C, is the result of the amalgamation of two such tubes, and 

 consequently in this chamber, or palcco-hystcron, the glandular lines 

 are symmetrically situated on each side. 



Any explanation, then, of the thyroid gland of Ammoccetes, must 



