ARTERIES OF FISHES. 489 



Cobitis, Nandus, Silurus, Batrachus, Gymnotus, Murcenophis, and 

 Mur&na are examples of genera in which it has not been detected. 

 In almost all other Osseous Fishes it is present, situated on each side 

 of the head, in advance of the dorsal end of the first biserial gill, 



o * 



under the form either of a small exposed row of vascular filaments, 

 like a uniserial gill (as in all Sciaenoids and many other Acantlw- 

 pteri, the Pleuronectidce, and the Lepidosteus, fig. 323, n) ; or, like 

 a vaso-ganglionic body, composed of parallel vascular lobes, and 

 covered by the membrane of the branchial chamber (as in Esox, 

 Cyprinus, Gadus, fig. 321, E,). In both cases the vein or efferent 

 vessel of the psetidobranchia becomes the ophthalmic artery, ib. k, 

 and the choroid ( vaso-ganglion,' when present, is developed from 

 it. The Sturgeon, like the Lepidosteus and Lepidosiren, has a 

 uniserial opercular gill, the homologue of the first so-called ' half- 

 gill ' of the Plagiostomes ; and, on the anterior wall of the ' spira- 

 cular canal,' a small vascular lamellate body receives arterialised 

 blood by a vessel sent off from the vein of the first biserial gill ; 

 which blood, after being subdivided amongst innumerable pinna- 

 tifid capillaries is collected again into the efferent vessel of that 

 body, and divides into the artery for the brain (encephalic), and 

 that for the eye (ophthalmic). The pseudobranchia is thus a kind 

 of f rete mirabile ' for both the cerebral and ophthalmic circulation 

 in the Sturgeon ] : in Osseous Fishes it stands in that relation to the 

 eye only, and is most generally associated with the more immediate 

 ophthalmic ' rete mirabile,' called f choroid gland,' fig. 216, o. 

 The pseudobranchia, in the Plagiostomes that have the spiracula, is 

 developed, as in the Sturgeon, on the anterior wall of each of those 

 temporal outlets from the branchial cavity : its ( vena arteriosa ' 

 supplies the eyes and part of the brain : it coexists in the Plagio- 

 stomes, Chima3roids, Sturgeons, and some Osseous Fishes, with 

 the vaso-ganglion supplied by vessels from the anterior branchial 

 veins, which lies bet ween the anterior basi-branchials and the sterno- 

 hyoid muscles. Besides the small nasal and orbital arteries, and 

 the hyo-opercular, from which the proper ophthalmic artery is 

 derived, the carotids are usually sent off from the ' circulus 

 aorticus.' In the Chimosra the carotids are transmitted directly 

 from the anterior branchial veins ; and, in the Pike, the artery of 

 the pectoral fins (brachial) is transmitted from the common trunk 

 of the two anterior branchial veins. In the Myxines an anterior, 

 as well as a posterior, aorta is continued from the common conflu- 

 ence of the branchial veins. In all higher fishes the posterior 

 aorta is the only systemic trunk so formed. 



1 xxi. pp. 41-67, 75. 



