VASCULAR SYSTEM OF FISHES. 



265 



of branchial development, taken chiefly from the observations of 

 Rathke *, must be premised. 



Five branchial arches and five branchial arteries, or vascular 

 hoops, are developed on each side in the embryo of all fishes above 

 the Dermopteri, as a general rule.f At first the trunk of the bran- 

 chial arteries simply bifurcates, the divisions passing round the pha- 

 rynx and reuniting on its dorsal surface, to form the aorta. Behind 

 this primary circle, which corresponds with the fold developing the 

 hyoid and mandibular arches, four additional arterial hoops are sent 

 off, which traverse, without further ramifications, the convex side of 

 the four antei'ior simple branchial ai'ches, and reunite above in the 

 aortic trunk {fig. 79, n, ri). If a sixth arterial arch is developed, cor- 

 responding with the fifth branchial arch, as its presence in the Lepido- 

 siren would indicate, it has not been observed, and must soon disappear 

 in most osseous fishes. In tliese the gills make their appearance as 

 leaflets budding out from the convexity of the four anterior branchial 

 arches, each leaflet supporting a corresponding loop of the branchial ar- 

 tery ; and, as the bifurcation and extension of the primary leaflets and 

 the pullulation of secondary laminae and loops proceed, the vascular 

 arch begins to separate itself lengthwise into two channels, traversed 

 by opposite currents, and thereby establishing an arterial and a 

 venous trunk in relation to the loojis and their vascular developments 

 on the branchial processes. In osseous fishes the primary arterial 

 arch, corresponding with the anterior or hyoid arch, developes either 

 a simple (uniserial) gill, or a plexiform, plumose, rudiment of a gill, 

 or both, or neither. In the Lepidosteus the anterior vascular arch 

 retains its primitive connection with the extremity of the branchi- 



arterial trunk, and developes on each side 

 a small uniserial pectinated gill (/^"^. 70. i ) 

 from the membrane clothing the inner 

 surface of the cerato-hyoid and pre-oper- 

 cular bones : the vein or efferent vessel 

 ((') of this gill goes to a smaller pecti- 

 nated organ iih. r), consisting likewise of 

 one scries of vascular filaments, which 

 agrees with the ' pseudo-branchia' of oilier 

 fishes in being supplied with arterial 

 blood. In the Sturgeon, the Lepidosiren, 

 and the Plagiostomes the representative 

 of the primary vascular arch has become, by partial bifurcation of 



Brancliion and pseudo-branchiii ; 

 Lepidosteus, Miiller. 



* CIX. ex. CXI. 



f The six-gilled Shark ( Hcxnnchus) and the sevcn-gillud Shark (Ilfptajic/nis) 

 arc among the few exceptions. 



