HIGHER CHORDATES 



273 



branchial openings 



spine 



spine 



SQUALUS ACANTHIAS 

 clasper 



claspers 



operculum pectoral fin 



HYDROLAGUS 



opening of clasper pouch 



Figure 16.10 External features ot the cartilaginous fishes: a shark, a ray, and a chimaeran. 

 (From Malcolm Jollie, Chordale Morphology, Reinhold Publishing Corp., New York, 1962.) 



cartilaginous skeleton. This skeleton distinguishes 

 these fishes from the true fishes, which possess a bony 

 skeleton. 



Sometimes the chimaeras and a fossil order are 

 joined in a Subclass Holocephali and the selachians 

 and fossil orders are placed in a Subclass Elasmo- 

 branchii (Gr. elasmos, thin plate + branchia, gills). 

 When Holocephali is considered a subclass, chimae- 

 ras are placed in an Order Chimaerae (Gr. chimaira, a 

 she-goat or mythical fire-spouting monster). 



Reproduction throughout the class involves inter- 

 nal fertilization; the males have accessory structures, 

 theclaspers, on the pelvic fins for this purpose. Shortly 

 after fertilization female chimeras and some female 

 sharks and rays lay "mermaids' purses," each purse 

 consisting of an egg in a horny capsule that is dis- 

 tinctive in the various species. These purses usually 

 are attached to objects; some egg capsules have "ten- 

 drils" that attach to such things as seaweeds. In spite 

 of these attachment devices, the egg cases are some- 

 times found in tidepools and along the seashore. 



Because fertilization is internal in the Chondrich- 

 thyes, it is only one step to the female's retaining the 

 eggs until hatching. This is the case with most sharks 

 and rays; hence, these cartilaginous fishes bear living 

 young. Whether eggs are laid or not, in most cases a 

 miniature shark or ray hatches without ever having 

 received nourishment from its mother, nourishment 

 being derived solely from the egg. However, in a few 

 sharks that bear their young alive the developing indi- 

 vidual does obtain sustenance from its mother. Con- 

 trary to popular belief, the mammals are not unique 

 in nourishing their embryos; such conditions are also 

 known in amphibians and reptiles, and even inverte- 

 brates. 



ORDER HOLOCEPHALI (Chimaeras) 



Diagnosis: gill slits covered by a membranous 

 structure (operculum); hence one external opening 

 leads into the four internal gill slits; grotesque form; 

 each jaw with teeth modified into a plate; without a 



