DISTRIBUTION OF SUBCUTANEOUS VESSELS IN GANOIDS 121 



an orifice (Fig. 15, B.), which leads cephalad into a pocket 

 through which water is ejected upon injecting either the second 

 or the third left branchial trunks. Neither from dissection nor 

 from the water injection method could the termination of the 

 fourth pair of branchial trunks be located. Undoubtedly, how- 

 ever, they are discharged into the inferior jugular vein. 



As shown in Fig. 15 the inferior jiigiilar vein in Lepisosteiis 

 arises in the branchial region from two branches, which unite 

 immediately behind and above the combined trunk formed by the 

 union of the last pair of afferent branchial arteries ; the right 

 fork, however, extends caudad for a short distance as a blind 

 sac. Thus formed, the main stem of the inferior jugular (/.j/. 

 K.), passes caudad above and parallel with the ventral aorta. 

 Dorsad of the bulbus, as already noted, it receives one or more 

 communications from below from the so-called dorsal pericardial 

 sinus ; after which it bears off obliquely to the left to discharge 

 itself in the left sinus venosus. It seems in Lefiisosteus that this 

 sinus is divided, and the opening into the auricle below is from 

 the left side (Fig. 15, S. Ven.O). Both sinuses receive a pre- 

 cava or ductus of Cuvier, but the hepatic and ventral veins, so 

 far as could be ascertained, terminated in the left precava. 



IV. DEDUCTIONS AND SUMMARY. 



From the preceding discussion it would seem that the subcu- 

 taneous vessels of the Ganoids occupy a position fairly well 

 between the generalized Selachians and the more specialized 

 Teleosts or Batrachians. There is a vast difference between 

 the character and distribution of these vessels in the Chondro- 

 ganoidea (the cartilaginous ganoids) and in the Holostei (the 

 bony ganoids). Polyodon as a representative of the former 

 group has non-sinus subcutaneous vessels, which in structure 

 are not very dissimilar to the corresponding veins, and in this 

 respect resemble the subcutaneous vessels of the Selachians, 

 which have been described by many authors as veins. While in 

 Lepisosteus, which is one of the bony Ganoids, many of the sub- 

 cutaneous vessels are thin-walled and decidedly sinus-like ; in 

 this respect resembling the conditions as found in the Teleosts 

 and Batrachians. 



