l;>() IVAN E. WALLIN 



l)()hi-n CS4) made the statement: 



the ^^rvixt (lilTcrcucc Ix'twccn th(> Selachian, Telcost and Ganoid bran- 

 chial a])iiaratus and that of the ]M'troTnyz()n consists therein that the 

 sill se])ta and lan)(>lhie (Kiem(n-l)l;i,tter und -l)lattchen) of the former 

 ar(> directed outwai'd while in the latter they are directed inward. 



He further states that this arrangement in the petromyzon 

 exists from the beginning. This interpretation of the gills of 

 petromyzon has been accepted in some of the textbooks on 

 Comparative Anatomy of Vertebrates The basis for this in- 

 terpretation is undoubtedly found in the position of the carti- 

 laginous gill bars, which form a complicated branchial basket 

 in the pharyngeal wall The branchial artery, however, is situ- 

 ated in the medial gill arch. From this medial gill arch the gill 

 septum extends caudo-laterally to its attachment in the lateral 

 wall. The gill lamellae are situated on the anterior and posterior 

 walls of the septum. The picture of a frontal section of the gills 

 in the ammocoetes is so much like the picture of a similar sec- 

 tion of the elasmobranch gills that it is difficult to consider 

 them as directed in opposite directions. The question resolves 

 itself nto a choice between the cartilaginous branchial bars 

 and the branchial aortic arches as a basis of interpretation. 

 It is evident that the branchial basket of petromyzon is a special 

 modification meeting the requirements of a specialized mode of 

 breathing due to the life habits of the adult. The position of 

 the cartilaginous gill bars must then be considered the result of a 

 migration from a more medial position. Moreover, the pres- 

 ence of the ciliated bands in the medial gill arches point to a 

 direct phylogenetic relationship to the gill arches of amphioxus. 

 If we consider the gills of the ammocoete as directed inward it 

 would be necessary to consider as the gill arch, the part of the 

 respiratory portion of the lateral branchial wall to which the gill 

 lamella is attached. This would be contrary to the arrangement 

 of the gills in all other chordate animals. 



