AL1MENTAR Y S YSTEM. 499 



Amphioxus it seems to arise as a pore in an ectodermic disc ; in other 

 cases it is a simple ectodermic invagination ; or it may owe its origin to 

 the coalescence of an anterior pair of gill-clefts innervated by the fifth 

 nerve. If the last interpretation be true, its origin illustrates that 

 change of function which has been a frequent occurrence in evolution. 

 But if the mouth arose from a pair of gill-clefts, and in some cases it 

 actually has a paired origin, then there must have been an older mouth 

 to start with. Thus Beard in his brilliant morphological studies dis- 

 tinguishes between " the old mouth and the new." The new mouth 

 is supposed to have resulted, as Dohrn suggested, from a pair of gill- 

 clefts ; the old mouth was an antecedent stomodaeum, of which the 

 so-called nose of Myxine and the oral hypophysis of higher forms may 

 be vestiges. This theory harmonises with the observations of Kleinen- 

 berg on the development of the mouth in some Annelids (Lopado- 

 rhynchus], in which the larval stomodasum is replaced by a paired 

 ectodermic invagination. 



The mouth cavity leads into the pharynx, on whose walls 

 there are the gill-clefts. Of these the maximum number is 

 eight, except in Amphioxus. If we exclude the hypo- 

 thetical clefts, such as those possibly represented by the 

 mouth, the first pair form the spiracles well seen in skates. 

 In the position of the spiracles the Eustachian tubes of 

 higher Vertebrates develop. In front of the spiracle there 

 is sometimes a spiracular cartilage, which Dohrn dignifies as 

 a distinct arch. The other gill-clefts are associated with 

 gills in Fishes and Amphibians, while in Sauropsida and 

 Mammals, in which there are no gills, four "visceral" clefts 

 persist as practically functionless vestigial structures. In 

 some cases their openings are very evanescent. The clefts 

 are bordered by the branchial arches, and supplied by blood 

 vessels and nerves. 



With the anterior part of the alimentary canal two 

 strange structures are associated the thyroid and the 

 thymus. 



The thyroid gland arises as a diverticulum from the ventral wall of 

 the pharynx. It may be single (as in some Mammals), or bilobed (as 

 in Birds), or double (as in some Mammals and Amphibians), or diffuse 

 (as in Bony Fishes). Only in the larval lamprey does it retain its 

 original connection with the pharynx, and is then a true gut-gland. 



As to its morphological nature, its mode of origin suggests com- 

 parison with the hypobranchial groove in Amphioxus and the endostyle 

 of Ascidians. 



Almost the only light which has been cast on the physiological nature 

 of the thyroid is from the pathological side. Goitre and Derbyshire 

 neck are associated with an enlargement and diseased state of this 



