106 ALLEN 



lymphatic openings into the hyo-opercularis sinus and the 

 cephalic trunk are larger than those into the inferior jugular, 

 barely make this view even tenable. 



Respiratory and Nutrient Branchial Arteries. These vessels 

 hold such an intimate relationship with the so-called branchial 

 lymphatic system, and are so different from the similar arteries 

 found in bony fishes, that a brief description of some kind seems 

 to be called for. 



With the exception of the last arch, which consists of a single 

 hemibranch, the respiratory system is practically the same for 

 the first three arches; so that the following description, which 

 is taken from the first arch of a 7o-pound Polyodon will answer 

 for all. The main afferent and efferent branchial arteries 

 (Fig. 17, A.Br.A. and E.Br.A.) maintain practically the same 

 positions and connections as in other fishes. An afferent fila- 

 ment artery (Fig. 17, A.Fil.A.) is given off distad from the 

 main afferent artery, to follow along the inner margin of each 

 filament ; its course lies in the connective tissue, close to the 

 filament, and its distribution within the filament is a great deal 

 more complex, than with other fishes that have been studied. 

 At regular intervals (Fig. 18) rather large, but short, cross 

 branches are given off, which are connected in an irregular 

 manner ; thus, forming a sort of secondary chain of arteries, 

 from which the so-called afferent transverse filament arteries 

 (Fig. 18. A.F. Tr.A.) take their origin. The latter vessels fol- 

 low a nearly parallel course with one another, until near the 

 filament septa, when each separates into from three to six 

 branches, each of which furnishes the venous supply for a 

 respiratory septum. In a 7<>pound Polyodon the faces of these 

 septa were about .1 mm. apart, which as shown by Figs. 18 

 arid i8a must contain about the finest capillary network that 

 exists. At the outer margin of each septum the respiratory net- 

 work is collected by a rather short efferent transverse filament 

 artery (Fig. 18, E.F.Tr.A.), which empties directly into the 

 efferent filament artery ', and all these vessels (Figs. 17 and 18, 

 E.Ftl.A.) pass proximad along the outer surface of the fila- 

 ments, to discharge themselves in, and form, the great efferent 

 branchial arteries (Fig. 17, E.Br.A.). 



