ARTERIES OF FISHES. 480 



Cohitis, JVaiuhts, Sihiriis, Batraclius, Gi/mnotiis, Murcenophis, and 

 Murcena are examples of genera in which it has not been deteeted. 

 In aUnost all other Osseous Fisiies it is present, situated on each side 

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

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

 like a uniserial gill (as in all Sciaiuoids and many other Acantho- 

 pter!, the Pleiirmicctida!, and the Lcpidostcvji, 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 Esux, 

 Cyprinus, Gadiis, fig. 321, r). In both cases the vein or efferent 

 vessel of the pseudol)ranchia 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 ojiercular gill, the homologue of the first so-called 'half- 

 gill ' of the Plagiostomes ; and, on the anterior wall of the ' sj)ira- 

 cular canal,' a small vascular lamellate body receives arterialised 

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

 which Idood, after being subdivided amongst innumeraljle pinna- 

 tifid capillaries is collected again into the efferent vessel of that 

 body, and divides into the artery for the lirain (encejihalic), and 

 that ibr tlie eye (ophthalmic). The pseudobranchia is thus a kind 

 of ' rete mirabile ' for both the cerebral and oplithalmio circidation 

 in the Stiirgeon ' : in Osseous Fishes it stands in that relation to the 

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

 ophthalmic 'rete mirabile,' called 'choroid gland,' fig. 2Ifi, o. 

 The pseudobrancliia, in the Plagiostomes that have the spiracnla, 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, ChiniKroids, Sturgeons, and some Osseous Fishes, witli 

 the vaso-wanglion supplied by vessels from the anterior branchial 

 veins, whichliesbetween the anterior basi-branchials and the sterno- 

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

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

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

 aorticus.' In the Chima^ra the carotids are transmitted directly 

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

 tlie iiectoral 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. 



' XXI. pp. 41-G7, 75. 



