138 FISHES. 



adjacent walls of tlie succeeding, a branchial arch with its 

 two series of radiating cartilaginous filaments is interposed. 

 Consequently the first and last pouch have one set of gill- 

 laminae only, viz. the first on its posterior and the last on its 

 anterior wall. The so-called spiracles on the upper surface of 

 the head of Chondropterygians are to be referred to in connec- 

 tion with the respiratory organs. They are the external 

 openings of a canal leading on each side into the pharynx, 

 and situated generally close to and behind the orbit. They 

 frequently possess valves or an irregularly indented margin, 

 and are found in all species during the embryonic stage, but 

 remaining persistent in a part only. The spiracles are the 

 remains of the first visceral cleft of the embryo, and in the 

 foetal state long branchial filaments have been observed to 

 protrude, as from the other branchial clefts. 



The Holocephali and Ganoidei show numerous deviations 

 from the Chondropterygian type, all leading in the direction 

 towards the Teleosteans. As a whole they take an inter- 

 mediate position between the preceding types and the Tele- 

 osteans, but they show a great variation among themselves, 

 and have in common only the imperfect separation of the 

 branchial sacs and the presence of a single outer branchial 

 aperture. 



In Chimcera the septum separating the branchial sacs is 

 confluent with the wall of the gill -cavity in a part of its 

 extent only, and still more imperfect is the separation of those 

 branchial divisions ia Ceratodus (Fig. 60). The other Ganoids 

 show no such division whatever. In Chimcera the first gill 

 is incomplete (uniserial), and belongs to the hyoid ; then fol- 

 low three complete gUls; the last, belonging to the fourth 

 branchial arch, being again incomplete. Acipenser, Scaphir- 

 hynchus, Lepidosiren, Protopterus, and Lepidosteus, possess like- 

 wise an anterior' incomplete gill (opercular gill), followed by 

 four complete gills in the Sturgeons and Lepidosteus, whilst in 



