266 



LECTURE X. 



the branchi-arterial trunk, a secondary branch, sent off by the artery 

 of the first branchial arch : but it nevertheless developes a simple 

 gill, of one series of filaments in the Lepidosiren {fig. 71. i), and of 

 one series of lamella? in the Plagiostomes : and this series is attached, 

 like the opercular gill of the Lepidosteus and Sturgeon, to the mem- 

 brane suppoi'ted by the hyoid arch. 



In most osseous fishes we recognise the reduced homologue of the 

 anterior primary vascular arch in that vessel {fig. 69. e.), Avhich is 

 continued from the venous or refluent division of the second primary 

 vascular arch ; not, as in the foregoing fishes, from the arterial 

 division of that arch, or from the branchial trunk. The ves&el in 

 question carries, therefore, arterial blood : it manifests its primitive 

 character by returning into the circulus aorticus (as at e',fig. 69.), but 

 now receives blood from it, and is called ' artei'ia hyo-opercularis :' 

 the pseudo-branchia, when present (as at r), is developed from it. 



In osseous fishes the four normal biserial pectinated gills are deve- 

 loped only from the four anterior branchial arches ; the fifth and last 

 arch has no gill developed from it, but is converted, as we have seen. 



into a pair of accessory jaws. In the 

 Lepidosiren, as in Ilexauchus, the fifth 

 arch supports a uniserialgill (fig.71. 6). 

 In the Planirostra, although the bran- 

 chial pecten is not developed from it, 

 yet the same kind of long slender 

 filamentary processes project inwards 

 from its concavity, as from that of 

 each of the anterior four pairs of 

 branchial arches. The five interspaces 

 between the hyoid arch and the five 

 branchial arches are originally exposed 

 on the sides of the head of the embryo 

 osseous fish ; the opercular and bran- 

 chiostegal appendages are later de- 

 velopments, and the single branchial 

 outlet is the resiilt of the formation of 

 the gill-cover. Thus the numerous 

 branchial apertures in the cartilaginous 

 fishes, like the substance of their skeleton, are retentions of embry- 

 onic structures. Very interesting arrests of development are also 

 found in bony fishes. We have seen that the primary vascular hoops 

 sweep over their respective arches without sending off' any branches, 

 the (svibsequently) branchial veins being in the embryo direct con- 

 tinuations of the branchial arteries. This primitive condition is per- 

 sistent in the fourth branchial arch of certain Mura:noid fishes of the 



Ciiciilating and rpspiratory Organs : 

 Lepidosiren. 



