PISCES. 425 



in most other Fish : four in Amia, six in Polypterus and Lepidosteus. The 

 walls of the ventricle are thick, and contain a lymphatic space in Teleostei 

 and Ganoidei. Muscular ridges project into its cavity, especially in Dipnoi, 

 and there is an incomplete septum in Lepidosiren. The conus is spirally 

 twisted in Dipnoi, and at the same time slightly folded upon itself. It 

 contains several rows of valves, variable in number, but most numerous in 

 bony Ganoidei, One row is especially prominent in Dipnoi, and in 

 Protopterus and Lepidopterus the valves in this row coalesce into a ridge. 

 It is opposed by another ridge of coalesced valves, and with these two ridges 

 there coalesces also in both Protopterus and Lepidosiren a septum which 

 cuts off the origins of the two anterior from the origins of the two posterior 

 pairs of aortic arches. The septum is incomplete in Ceratodus. The effect 

 of this arrangement, coupled with the peculiar structure of the heart, is to 

 send into the two first pairs of arches in Ceratodus a current of mixed 

 arterial and venous blood ; of pure arterial in the other two Dipnoans 

 (Boas). 



A sub-branchial artery or ventral aorta springs from the conus, or from 

 the ventricle in Teleostei, where its entrance is guarded by a pair of valves, 

 remnants of the anterior row of the conus. The commencement of this 

 vessel is dilated, and is termed bulbus aortae. It gives origin to the aortic 

 arches, or branchial arteries, which are placed at some distance apart one 

 from the other except in Dipnoi, where their roots are close together, as in 

 Amphibia. There are five branchial arteries in Elasmobranchii, Holocephali, 

 chondrostean Ganoidei and Lepidosteus ; the first supplies the hyoidean, or 

 opercular gill, the remainder the gills of the branchial arches. Hexanchus 

 and Heptanchus have respectively one and two additional arches. The 

 hyoidean vessel springs from the first branchial artery, not from the ventral 

 aorta, in Polypterus and Protopterus, and carries venous blood ; but it 

 supplies no gill in the first-named Fish. The hyoidean vessel is an artery 

 carrying arterial blood and supplying the pseudobranchia in Teleostei, and 

 takes origin from the ventral end of the first branchial vein. In this order 

 there are typically four branchial arteries. The branchial veins fall dorsally 

 either into a median vessel or into a right and left epibranchial artery, 

 which fuse into a median vessel. This vessel in either case is the sub- 

 vertebral aorta, which is continued to the tip of the tail as the caudal 

 artery. The common carotids arise from the dorsal extremity of the 

 hyoidean vein or, in Teleostei, artery (supra), which is connected dorsally to 

 the subvertebral aorta or epibranchial artery. The two carotids are con- 

 nected across the base of the skull (except in Dipnoi) by a vessel, thus 

 making a circulus cephalicus. The internal carotids arise from this con- 

 necting vessel. The venous blood returns to the sinus venosus by a right 

 and left ductus Cuvieri, formed by the fusion of the jugular and subclavian 

 veins with the posterior cardinal veins (p. 352). The right ductus in Lepi- 



