PE TROM YZONTIA 



53 



slit. The groove is carried back along the floor of the pharynx into 

 the opening of the thyroid gland. This gland develops as a mid- 

 ventral outgrowth of the pharynx, acquires a lumen of considerable 

 size, and along its folded walls become differentiated four rows of 

 mucous cells (Fig. 36). In fact, the whole structure bears a 

 striking and unmistakable resemblance to the endostyle of the 



th 



Fio. 35. 



Transverse section of the gill-region of an Ammocoete larva, somewhat diagrammatic. (Partly 

 after Alcock.) a.c, anterior cardinal vein ; of, afferent artery ; 6.6, branchial basket ; d.a, dorsal 

 aorta ; d.., dorsal blood-sinus ; ef, efferent artery ; g, gill-lamella ; g.o, gill-opening ; l.n, lateral- 

 line nerve; n, nerve-cord; nt, notochord ; th, thyroid gland; v.a, ventral aorta; v.n, vagus 

 nerve ; v.s, ventral blood-sinus. 



Tunicate and Cephalochorda, with which it is no doubt homologous 

 (\\. Miiller [30 7 a]). It may also be mentioned that the liver is 

 composed of a mass of branching tubules, much more distinct than 

 in the compact liver of higher vertebrates ; in the adult it undergoes 

 fatty degeneration, and the gall-bladder is lost. The paired eyes 

 lie hidden deep beneath the skin. The pericardium opens widely 

 into the abdominal coelom (Fig. 37, C) ; on its dorsal wall lies the 

 pronephros. The median fin is continuous. 



