DIGESTIVE SYSTEM 143 



right of the latter, between it and the duodenum. The ascending 

 branch of one loop runs side by side with the descending branch of 

 the next following one. 



II. Anticoelous. — The second and third loops are closed and 

 sharply alternating ; the second is left-, the third is right-handed ; 

 the second lies dorsally, consequently its ascending branch runs side 

 by side with that of the third. 



III. Plagiocodous. — The second and often more loops are doubled 

 or turned over with the apices like a horseshoe, giving the loops, 

 which are generally open, an irregular or convoluted appearance. 



IV. Pericoelous. — The second loop is left-handed, open, and 

 encloses the third which is generally straight and closed. This 

 formation is of especial interest, because it leads quite gradually to 

 the 



V. Cydocodous formation by the conversion of the second and 

 third loops into one left-handed spiral. Such a conversion of the 

 second and third loops into a spiral occurs in the Limicolse, LaridcB, 

 and Golumhds. Each of these families possesses some genera in which 

 the spiral is still represented by long, oval, concentric turns, and 

 even some genera which still exhibit the pericoelous type with the 

 two loops in question still separate, distinct, and more or less 

 straight. 



Not every spiral, however, is formed by the concentration of 

 two loops. In many instances a spiral is produced by one loop 

 being curled upon itself, its apex then forming the centre of the 

 spiral. To the apex is attached the diverticulum cajcum vitelli ; 

 this shews that this spiral is produced by the primitive fold of the 

 embryonic mid-gut. 



Such is the case in all the Passeres, and since there are only 

 three folds formed by the whole gut, the spiral represents the 

 middle or second fold ; hence this arrangement may be distinguished 

 as mesogijrous. The number of turns in such a spiral depends 

 directly upon the length of the intestine ; while in the short-gutted 

 Sylvise the spiral is just indicated, there are in the Sparrow (with 

 an intestinal length of 21 cm.) 1| direct and 1 retrograde turn, 

 and in Pinicola enudeator (which possesses an intestine of 99 

 cm. in length) there are many direct turns. 



It is clear that with an original number of only four loops, the 

 conversion of the two middle ones into a single spiral will cause 

 such birds as certain Limicolse, Laridx, and Colunibse likewise to 

 assume the mesogyrous feature ; but the position of the diverticle 

 on the original third loop, and the relations of these birds, e.g. 

 Charadrius and Sterna, shew that this mesogyrous formation has 

 been brought about in a way different from that of the Passeres. 



Lastly, the distal portion of any loop originally straight may be 

 coiled up into a spiral, while the rest of the loop remains straight. 



