Chapter 4

• Gnathostomata


  Vertebrates having jaws, the lower is mobile
• Gnathostomes


  • Presence of a jaw
  • Constant presence of two nares and two nasal pits
  • Presence of three semi-circular canals (sensorial organs of equilibrium)
  • Fundamental possession of two pairs of lateral limbs
  • Separation of the lateral muscle of the trunk into two parts due to the presence of the transversal myoseptum
• Gnathostomata is divided into two super-classes

  • Super-class Pisces
  • Super-class Tetrapoda
• Super-class Pisces


  • Class Chondrichthyes
  • Class Osteichthyes
• Super-class Tetrapoda


  • Class Amphibia
  • Class Reptilia
  • Class Aves
  • Class Mammalia
• Class Chondrichthyes


  Cartilaginous fish, mostly marine; bone entirely absent; notochord reduced but present in adult; scales placoid; tail heterocercal; nostrils paired; internal nares absent; without air sac; with lateral line system; sexes separate; fertilization internal; development direct; oviparous; ovoviviparous or viviparous

  
• Chondrychthyes are divided into two sub-classes

  • Selachii (sharks and rays)
  • Holocephalii (Chimeras)
• Sub-class Selachii (Elasmobranchii)


  • Possession of 5-7 externally visible pairs of slit-gills
  • With spiracles
  • Without operculum
  • Presence of numerous replaceable denticles (numerous teeth)
  • Hyostylic articulation of the lower mandible with the cranium
  • Upper jaw not fused with the brain case
  • Dorsal fin non erectile
• Orders of Selachii


  • Squaliformes (sharks)
  • Rajiformes (rays)

  
• Sub-class Holocephalii (bradyodonts)


  • Without spiracles
  • Four pairs of gills, with operculum
  • Teeth fused as six pairs of plates
  • Upper jaw fused to brain case (autostylic)
  • Cranium mobile on vertebral column
  • Dorsal fin erectile
  • Scales absent in adult
  • Tail whiplike
  • Without ribs or cloaca
  • Males with claspers
• Spiny dogfish shark


  The most abundant shark, with mildly poisonous dorsal fin spines, often studied in scientific laboratories, used extensively as food, fertilizer, pet food, and liver

  
• External morphology of spiny dogfish shark

  • Average size 0.9-1.2 m long
  • Eats mostly fish, squid and octopus
  • Graceful and streamlined body shape built for fast, long distance swimming
  • Body divided into head, trunk, and tail
  • Dark gray above, almost white below
  • Head flattened ventro-dorsally, body compressed bilaterally
  • Lateral line - light-colored horizontal stripe made of tiny pores sensitive to water movement and pressure
  • Double dorsal fin, anterior larger than posterior, with two poisonous spines
  • Pointed snout (rostrum) helps overcome water resistance
  • Prominent eyes similar to human eyes
  • Large spiracle openings posterior and dorsal to eyes
  • Five external gill slits
  • Paired pectoral and pelvic fins
  • Heterocercal caudal fin
  • Mouth on underside, with several rows of sharp, pointed teeth
  • Nares (nostrils) on underside of rostrum
  • Ampullae of Lorenzini - pores sensitive to temperature, pressure, electrical fields, salinity
  • Males have claspers for internal fertilization
  • Cloacal opening receives intestinal, urinary and genital products
  • Muscles in W-shaped myomeres for side-to-side swimming motion

  
• Digestive system


  • Teeth in rotating rows to replace worn ones
  • Largest organ is the liver, rich in oil for buoyancy
  • Thick muscular esophagus, J-shaped stomach with rugae, pyloric sphincter
  • Duodenum, valvular intestine with spiral valve, colon, rectal gland for osmoregulation
  • Cloaca collects products of colon, urogenital ducts
• Respiratory system


  Spiracles bring water to gills, which have rich blood supply for gas exchange
• Circulatory system


  • Pericardial cavity contains heart and major blood vessels
  • Ventricle pumps blood through conus arteriosus to gills and body
  • Atrium pumps blood to ventricle, blood enters through sinus venosus
• Bioecology of sharks


  • Poison organs - stingrays have venomous spines
  • Electric organs - electric rays can produce electric discharge to stun prey