272 



SUBPHYLUM VERTEBRATA: 



opercular fold 



ICHTHYOSTEGA 



Figure 16.8 Restoration of Icthyosiego, a Devonian stem omphibian. (From Malcolm Jollie, 

 Chordate Morphology, Reinhold Publishing Corp., New York, 1962.) 



like larvae that spend several years partly buried in 

 the stream bottom, a place suitable for their filter- 

 feeding habits. Two possibilities occur at the time of 

 larval metamorphosis: some become predaceous or 

 parasitic adults that pursue an active life for some 

 time; others become adults that neither feed nor 

 grow, but after a few months reproduce and die. 



CLASS CHONDRICHTHYES (Cartilaginous Fishes) 



Diagnosis: streamlined, cylindrical to flattened, 

 perhaps to 50 feet long; skin tough and covered with 

 minute, unique (placoid) scales which also are modi- 

 fied into teeth; skin with many slime or mucus 



glands; skeleton of gristle or cartilage; tail and two 

 median dorsal fins present; appendages present, 

 paired front or pectoral and paired rear or pelvic fins; 

 jaws and fin rays present; five to seven pairs of gills 

 and gill slits (usually exposed); sexes separate, fertili- 

 zation internal, eggs laid or held and young born 

 alive; development direct, so no larval stage; United 

 States species mostly marine (Figure 16.10). 



The Chondrichthyes include two living orders, the 

 chimeras (sometimes also called ratfishes or ghost 

 fishes) with no recognized suborders, and the selachi- 

 ans with two suborders, the sharks and rays. As a 

 group the Chondrichthyes are called cartilaginous 

 fishes, a designation referring to their gristle or 



nasohypophyseal opening 



dorsal fin 



caudal fin 



buccal funnel-^"^ — ' bronchial opening 



PETROMYZON OR LAMPETRA 



nasohypophyseal 



opening _ 



tentacles 



phoryngocutoneous duct (on left side) 



EPTATRETUS 



opening of slime gland 



branchial opening 



Figure 16.9 Some external features of a lamprey (top) and o hagfish (bottom). (From Malcolm 

 Jollie, Chordate A/lorpho/ogy, Reinhold Publishing Corp., New York, 1962.) 



