ALIMENTARY SYSTEM. 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 stomodzum, of which the 
so-called nose of AZyxine 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 (Zopado- 
rhynchus), in which the larval stomodzeum 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 ¢hyroid 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 
