282 FISHES 



CHAP. 



behind the last gill-sac of the left side, and opens externally 

 with the common external branchial aperture of that side. 



In Bdellostoma there are usually six or seven pairs of gill- 

 sacs, but some species have ten or even fourteen pairs. 1 They 

 agree with those of the Lamprey in having independent external 

 apertures, but resemble the corresponding organs in Myxine in 

 opening directly into the pharynx. An oesophageo- cutaneous 

 duct is also present/' 



In the Holocephali there are but four branchial clefts, the 

 fifth cleft being closed. Spiracles are absent in the adult, 

 although present in the young of Chimaera, The branchial 

 lamellae resemble those of Elasmobranchs, but the inter- 

 branchial septa are somewhat shorter, so that the lamellae 

 project slightly beyond their -outer margins (Fig. 164, B). A 

 hyoidean hemibranch is present. A noteworthy feature is the 

 development of a cutaneous fold from the outer surface of the 

 hyoid arch, which grows backwards over the gill-clefts, and, 

 uniting above and below with the body-wall, terminates in a free 

 posterior margin, just behind the last gill-cleft. By the growth 

 of this opercular fold the gills become enclosed in a spacious 

 branchial cavity, and the clefts communicate with the exterior 

 through a slit-like opening between the free margin of the fold 

 and the body-wall. 



The reduction in the extent of the inter -branchial septa which 

 is initiated in the Holocephali is carried to a still further extent 

 in the Teleostomi. Commencing with the Chondrostei, and 

 passing thence to the more specialised Teleostei, the septa become 

 gradually reduced in length, and the branchial lamellae project 

 freely beyond their outer margins to an increasing extent. 



This modification, least marked in Acipenser (Fig. 164, C) 

 and Polyodon, attains its maximum in the Teleosts (Fig. 164, 

 D and E), where the branchial lamellae take the form of a 

 double series of free filaments disposed along the convex outer 

 margin of each branchial arch, and attached by their bases only 

 to the reduced and inconspicuous septa. As a general rule each 

 of the first four arches supports two hemibranchs, 3 forming a 



1 See p. 423. 



- Howes (P.Z.S. 1893, p. 730) has described certain remarkable variations in 

 the respiratory organs of Petromyzon and Myxine. 



3 In certain Teleosts more or fewer of the branchial arches may lose their gills. 

 This reduction attains its maximum in the singular Indian amphibious Fish, 



