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.’ They 
agree with those of the Lamprey in having independent external 
apertures, but resemble the corresponding organs in Myaine in 
opening directly into the pharynx. An oesophageo-cutaneous 
duct is also present.” 
In the Holocephah 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 Holocephah 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,? forming a 
' See p. 423. 
? Howes (P.Z.S. 1893, p. 730) has described certain remarkable variations in 
the respiratory organs of Petromyzon and Mywine. 
’ 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, 
