KESPIRATION. 137 



gills are lodged in a cavity, but during the embryonic stage 

 the Chondropterygians have the gill-lamiuie prolonged into 

 long filaments projecting beyond the gill-cavity (Fig. 58), and 

 in a few young Ganoids external gills are superadded to the 

 internal. 



In BrancJiiostoma the dilated pharynx is perforated by 

 numerous clefts, supported by cartilaginous rods (Fig. 29, h). 

 The water passes between these clefts into the peritoneal 

 cavity, and makes its exit by the porus abdominalis situated 

 considerably in advance of the vent. The water is propelled 

 by cilia. 



In the Cyclostomes the gills of each side are lodged in a 

 series of six or more antero-posteriorly compressed sacs, 

 separated from each other by intervening septa. Each sac 

 communicates by an inner duct with the oesophagus, the 

 water being expelled by an outer duct. In Bdcllostoma each 

 outer duct has a separate opening, but in Myxine all the outer 

 ducts pass outwards by one common gill-opening on each side. 

 In the Lampreys the ducts are short, the outer ones having 

 separate openings (Fig. 2, p. 39). The inner ducts lead into 

 a single diverticulum or bronchus, blind behind, situated 

 below the oesophagus, and communicating in front with 

 the pharynx, where it is provided with two valves by which 

 the regurgitation of the water into the buccal cavity is 

 prevented. 



The same type of branchial organs persists in Chondrop- 

 terygians, which possess five, rarely six or seven, flattened 

 pouches with transversely plaited walls. The septa between 

 them are supported by cartilaginous filaments rising from the 

 hyoidean and branchial arches. Each pouch opens by a cleft 

 outwards, and by an aperture into the pharynx, without inter- 

 vening ducts. The anterior wall of the first pouch is sup- 

 ported by the hyoidean arch. Between the posterior wall of 

 the first and the anterior of the second sac, and between the 



