486 ANATOMY OF VERTEBRATES. 



hyo-opcrcnlaris : ' the pseudo-branchia, when present, as at 

 fig. 321, R, is developed from it. 



In Osseous Fishes the four normal biscrial pectinated gills are 

 developed 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 Hexanchus, the fifth arch supports a uniserial gill, fig. 324, 6. 

 In the Planirostra, although the branchial 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 branchiostegal appendages are later developements, 

 and the single branchial outlet is the result of the formation of the 

 gill-cover. Thus the numerous branchial apertures in the carti- 

 laginous fishes, like the substance of their skeleton, are retentions 

 of embryonic structures. Very interesting arrests of developemcnt 

 are also found in bony fishes. We have seen that the primary vas- 

 cular hoops sweep over their respective arches without sending off 

 any branches, the (subscf[ucntly) branchial veins being, in the 

 embryo, direct continuations of the branchial arteries. This 

 primitive condition is persistent in the fourth branchial arch of 

 certain Muramoid fishes of the Ganges, Monopterus, Symbranchus\ l 

 it is persistent in the first and second branchial arches of the eel- 

 like Lepidosiren, fig. 324. 2, 3. Such arches are, therefore, gill- 

 less, and a certain proportion only of the blood transmitted from 

 the heart is aerated in the gills : about one fourth, e. g. in Mono- 

 pterus, goes directly to the aorta in its venous state ; a larger 

 quantity would pass into the roots of the aorta, fig. 312, o, o, and 

 mix with the general circulation in the Lepidosiren, were no part 

 of the current diverted by the vessels /, /', into the lung-like 

 modification of its air-bladder. 



A tuft of filaments, supporting each a single vascular loop, and 

 covered with non-ciliate epithelium, extends from each branchial 

 plate, protruding from the outer slit, in the embryo of the Plagio- 

 stomes ; 2 and a similar tuft also extends from the spiracle in those 

 species which possess it, e. g. Mustelus and Acanthias ; but 

 these preliminary branchial organs soon disappear. 4 Three seem- 

 ingly analogous filaments are retained on each side, for a longer 

 period, in the Lepidosiren annectens\ but lose that vascular and 



1 cxix. 2 xx. vol. v. p. 72. 3 lxix. p. 88, pi. 14. 



1 LXXXII. CXXV. CX1II. p. 97. 



