PTERYLOSIS 19 



apply. But the todies were found to be birds with a tufted 

 oil gland and with large caeca. 



It has been pointed out that when the oil gland has a 

 tuft of feathers upon its apex the rest of the gland is un- 

 feathered, and that, on the contrary, when the tip is nude 

 the general surface of the gland is feathered. The oil gland 

 is, so far as we know, a structure special to birds : it is, indeed, 

 the only purely external glandular apparatus that exists in 

 them. It is therefore possible, if not probable, that the 

 organ first arose in the class — that it is not an inheritance 

 from any ancestor. On this view it is quite possible that 

 the absence of the oil gland may not be always due to its 

 disappearance ; birds without oil glands may or may not 

 have lost them. It seems very likely, for example, that the 

 usual absence of this structure among the struthious birds 

 is rather a primitive than a secondary character. If this 

 view of the matter is justifiable, the presence of a tuft may 

 also be, in some cases at least, secondary ; for it is certainly 

 a specialisation that may have appeared after the oil gland 

 was fully developed. 



Alimentary Canal 



The tongue ' of birds is one of the most variable organs 

 as to size and texture. In Plotus, for example, it is 

 practically altogether absent (see fig. 5) . When present it 

 is larger or smaller, more or less fleshy. The long, thin, . 

 horny tongue of the toucans characterises those birds; the 

 parrots (see fig. 6) have a thick and fleshy tongue ; naturally 

 the organ is of more use to the latter than to the former. 

 A very remarkable modification of the tongUe, seen in birds 

 quite remote in the scale, is the pulling out of the free end 

 into a tuft of fine fibres ; this is associated with the capture 

 of honey oi- minute insects from the corollas of flowers ; it is 



■ ' C. S. MiNOT, ' studies, on the Tongue of Eeptile's and Birds,' Arm. Mem. 

 Boston 'Soc. Nat. Hisi. 1880 ; Nitzsoh-Giebel, ' Die Zunge der Vogel,' &c., 

 Zeitschr. f. d. ges. Naturw/xi. 1858, p. 19 ; Ludwig, Peinz v. Bayekn, Zur 

 Anatomic der Zunge (Miinohen, 1884). 



o2 



