SALPA IN RELATION TO EVOLUTION OF LIFE. 191 



W. Miiller was the first to point out the homology of the tunicate 

 endostyle with the vertebrate thyroid gland, and this homology has 

 been established beyond the possibility of doubt by Schneider's 

 discovery that the thyroid body of ammocoetes is a slime gland 

 with an opening into the pharynx near the mouth, and that on 

 each side of this opening a ciliated furrow or peripharyngeal band 

 runs upwards on the. inner wall of the pharynx, just in front of 

 the first gill-slit, to its dorsal middle line, where the two unite to 

 form an epipharyngeal band or dorsal lamella which runs back- 

 wards to the oesophagus. Even more conclusive proof of this 

 homology is afforded by Dohrn's account (Studien, VIII) of the 

 histological structure of the pharyngeal gland of ammocoetes, for 

 his studies show on the one hand a most complete fundamental 

 identity with the very peculiar and characteristic histological struc- 

 ture of the tunicate endostyle, and they also, on the other hand, 

 prove its identity with the vertebrate thyroid gland, by showing 

 that, as development progresses, it is cut up by ingrowths of con- 

 nective tissue into the isolated follicles which are so characteristic 

 of the thyroid gland. Still further confirmation is furnished by 

 Dohrn's discovery in the torpedo embryo (Studien, VIII, p. 60) of 

 two eudodermal grooves which run from the ventral margins of 

 the spiracles to the ventral middle line of the pharynx, to end at 

 the median unpaired thyroid invagination in such a way as to prove 

 that they are rudimentary peripharyngeal grooves. 



This most remarkable homology can no longer be questioned. 

 The simplest explanation, and the one which first presents itself, is 

 the one which Miiller advances, that the common ancestor of the 

 tunicates and of the other chordata, possessed this system of organs 

 in the form in which we now find it in the tunicates, and that while 

 all the jawed vertebrates have inherited the ventral pharyngeal 

 gland, it has been turned in them to some new use, as yet undis- 

 covered by the physiologists, and has lost its primitive connection 

 with the pharynx and its functional relation to the mouth, and has 

 become a ductless aggregation of follicles far back in the throat. 



I have tried to show that the structure and anatomical relations 

 of this system of organs in the tunicates are quite consistent with 

 the view that it was originally acquired for the purpose which it 

 now serves, the capture of food. • 



