196 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



who proved that it is a pharyngeal gland with excretory cells to produce 

 slime, and with ciliated cells to drive the slime out through the long, 

 narrow, slit-like duct into the pharynx. Fol also showed its true relation 

 to the ciliated peripharyngeal bands and dorsal lamella, and proved by 

 simple but conclusive experiments that these organs are co-ordinated 

 parts of a single system, which has for its function the capture of the 

 microscopic floating food which enters the mouth with the water. 



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

 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 Tipwards 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 backwards 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 structure of the 

 tunicata endostyle, and they also, on the other hand, prove its identity 

 with the vertebrate thyroid gland, by showing that, as development pro- 

 gresses, it is cut up by ingrowths of connective 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 endodermal 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 verte- 

 brates have inherited the ventral pharyngeal gland, it has been turned 

 in them to some new use, as yet undiscovered 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. 



