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The Pseudobranchial and Carotid Arteries in Polyodon spathula. 



By Edward Phelps Allis jr., Menton. 



With one Figure. 



(Schluß.) 



In Polyodon, it may here be stated, the eye-muscles are innervated 

 as they are in Amia (Allis, 1897), Lepidosteus (Allis, 1910) and 

 Acipenser (Corning, 1900), these three ganoids being the only ones 

 that have been properly investigated in this respect. This method of 

 innervation of these muscles is not however a strictly ganoidean 

 characteristic, for in Ameiurus they are also so innervated (Work- 

 man, 1900). 



There are, in Polyodon, four ejäerent branchial arteries on each 

 side, the three posterior of which empty into the median dorsal aorta, 

 while the anterior one empties into what, in the nomenclature I have 

 employed, is the lateral dorsal aorta of its side. Each of these 

 arteries begins at the ventral end of its arch as two vessels, of equal 

 size in sections, one lying on either side of the single afferent artery. 

 At about the middle of the length of the ceratobranchial, the posterior 

 one of these two vessels in each arch passes internal to the afferent 

 artery and joins the anterior vessel, the two then forming the single 

 efferent artery of the arch, which soon comes to lie directly internal 

 to the afferent one. As the union of these two efferent vessels, in 

 each arch, takes place in the lower (distal) half of the entire arch, 

 the commissure there represented must be the homologue of one of 

 the two ventral ones, and not of the dorsal one, of the descriptions 

 of the Scylliidae and of Mustelus. The dorsal commissures are not 

 found regularly developed between any of the arches in Polyodon, but 

 from the efferent arteries in the third and fourth arches branches are 

 given off, one in each arch, and these branches not only supply the 

 dorsal portion of the related gill but meet and become continuous 

 above the fourth gill cleft. These branches are not found in the other 

 arches, but from the efferent artery in the first arch, near its dorsal 

 end, a large branch is sent upward and backward, internal to the 

 thymus, along the lateral wall of the skull. In Allen's drawings, 

 the two efferent vessels that I find in each arch are shown in the 

 first two arches only, the efferent arteries in the two posterior 

 arches being shown single throughout their entire length. 



