22 STRUCTURE AND CLASSIFICATION OF BIRDS 



or temporary, foreshadows the fully developed crop of the 

 birds mentioned. 



The stomach ' consists of two compartments following 

 each other, the glandular proventriculus and the more mus- 

 cular gizzard. The proportions of these two segments of 

 the stomach vary, and both are much reduced in the 

 ^ hoatzin, whose crop appears to take on the function of a 

 gizzard. The proventriculus is usually, but not always, 

 separated by a marked constriction from the gizzard, and has 

 a patch of large glands which generally forms a band lining 

 the upper part of the sac and continuous right round it ; to 

 a proventriculus in which the glandular patch is disposed in 

 this fashion the term ' zonary ' is applied. More rarely the 

 patch of glands is a single oval or round patch not continu- 

 ous round the proventriculus, or there may be two such 

 patches. In Plotus anhinga the two patches of proven- 

 tricular glands are contained in a special diverticulum 

 of the proventriculus. In Tantalus ibis Mitchell^ has 

 described a remarkable divergence from the usual structure 

 of the proventriculus. In this bird the glandular areas are 

 two, as in other storks. Above these is a row of crypts, 

 which are partly glandular and partly lymphatic, and are 

 believed to be organs for the absorption of water. Among 

 the Steganopodes and in other birds the proventriculus is 

 much larger than the gizzard, which follows. In certain 

 tanagers this state of affairs culminates in the apparent 

 absence of the gizzard as a distinct structure (see below). 

 Lund and Foebbs have mentioned a number of tanagers in 

 which this occurs. The gizzard is more muscular in grain- 

 eating and in some other birds than it is in flesh- and fish- 

 eating birds. It is strong and hard and lenticular in form in 

 the Galli, Ralli, &c., bag-like and soft-walled in the heron, 

 &c. The lining of the gizzard undergoes a remarkable 

 modification in certain pigeons {q.v.), where it may be even 

 ossified. 



' ' On the Proventrioular Crypts of Fseudotantalus ibis,' P. Z. S. 1895, p. 271. 



'' A comprehensive work upon this organ is that of Cazin, Ann. Sci. Nat. (7), 

 iv. 1887, p. 177. See also the same, ' Structure et M6canisme du GSsier des 

 •Oiseaux,' Bull. Soc. Philom. 1888, p. 19. 



