GILLS. 



141 



system these organs have lost those functions, and appear in 

 the adult fish as retia mirabilia, as they receive oxygenised 

 blood, which, after having passed through their capillary system, 

 is carried to other parts of the head. In Palseichthyes the 

 pseudobranchia is a rete mirabile caroticum for the brain and 

 eye ; in Teleosteans a rete mirabile ophthalmicum only. 

 Pseudobranchise are as frequently absent as present in Chon- 

 dropterygians as well as Teleosteans. As to the Ganoids, they 

 occur in Ccratodus, Acipenser, 

 Polyodon, and Zepidosteus, and 

 are absent in Lepidosiren, Pro- 

 topterus, Scaphirhynchus, Foly- 

 pterus, and Amia. 



In Chondropterygians and 

 Sturgeons the pseudobranchise 

 are situated within the spiracles ; 

 in those, in which spiracles have 

 become obliterated, the pseudo- 

 branchiae lie on the suspen- 

 sorium, hidden below cellular 

 tissue ; but pseudobranchiae are 

 not necessarily eo-existent with 

 spiracles. In the other Ganoids 

 and Teleosteans the pseudo- 

 branchise (Fig. 60, h) are within 

 the gill-cavity, near the base of 

 the gill-cover ; in Ceratodus even 

 rudiments of the gill-rakers 

 {x, x") belonging to this embryonic gill are preserved, part 

 of them ix") being attached to the hyoid arch. Pseudo- 

 branchise are frequently hidden below the integuments of 

 the gill- cavity, and have the appearance of a glandular body 

 rather than of a gill. 



[See Miiller, " Vergleichende Anatomie des Gefass-systems der Myxinoi- 

 den ;" and "Ueber den Bau und die Grenzen der Ganoiden."] 



Fig. 60.— Gills of Ceratodus. 



Arcus aortae ; gl, Glossohyal ; 

 cli, Ceratohyal ; u, Attachment of 

 the first gill to the walls of the 

 gUl-cavity ; h, Pseudobranchia ; 

 X' , x", two series of gill-rakers be- 

 longing to the Pseudobranchia. 



