Summary Fishes

FISHES:




SUPERCLASS AGNATHA:
Living jawless fishes:
 2 classes: Myxini and Petromyzontida  Small brain, 10 pairs of cranial nerves
 No jaws, internal ossification, scales or paired fins  Poor eyes, semi-circular canals present
 Pore-like gill openings and an eel-like body form  Sexes separate
 Fibrous and cartilaginous skeleton  External fertilisation
 Vertebrae reduced or absent  5-16 pairs of gills
 No distinctive stomach  Heart with a sinus venosus, atrium and ventricle

 Myxini: hagfishes
 Marine group
 Feed on annelids, molluscs, crustaceans
and dead or dying animals. They enter a
dead or dying animal through by digging
into the body. Using toothed, keratinized
plates on its tongue that fold together in
a pincer-like action, they rasp bits of
flash from its prey. For extra leverage
they tie a knot in its tail and pass the
knot forward along its body
 Almost completely blind. Well-developed
sense of smell and touch
 Can generate large amounts of slime
when disturbed
 Their body fluids are in osmotic
equilibrium with seawater
 Low-pressure circulatory system served by 2 accessory hearts and a main heart positioned behind the gills
 Females produce a few large yolky eggs which take 5 months to hatch. No larval stage

,  Petromyzontida: lampreys
 Some are parasitic
 Swim up streams to spawn in spring/summer.
Males build a nest and are joined by females. At
spawning, with the female attached to a rock, a
male attaches to the dorsal side of her head. As
eggs are shed into the nest they are fertilised by
the male. Adults die after spawning. Eggs hatch
after 2 weeks, releasing small larvae called
ammocoetes
 Sharp keratinized teeth rasp flesh
 To promote the flow of blood they inject an
anticoagulant into the wound

CLASS CHONDRICHTHYES: (Cartilaginous fishes)

Characteristics of Chondrichthyes:
 Body dorsoventrally compressed  Well-developed brain, 10 pairs of cranial nerves
 Caudal fin heterocercal or diphycercal  Senses of smell, lateral-line, vision and
 Paired pectoral and pelvic fins electroreception, 3 pairs of semi-circular canals
 Skin with placoid scales or naked  Sexes separate, internal fertilisation - claspers
 Cartilaginous skeleton  Oviparous or viviparous
 Notochord reduced  Excretory system of kidneys
 Distinct vertebrae  5-7 pairs of gills
 Jaws with polyphyodont teeth  No swim bladder or lungs
 Large stomach  Heart with a sinus venosus, atrium, ventricle and
 Intestine with spiral valve conus arteriosus. Single circulation
 Large oil filled liver

Subclass Elasmobranchii: sharks, skates and rays Eg. Dogfish sharks
 Heterocercal tail
 Paired pectoral and pelvic fins supported by
appendicular skeletons, 1-2 median dorsal fins, a
median caudal fin. Some have a median anal fin
 In males, medial part of pelvic fin modified to
form a clasper used in copulation
 Paired nostrils are associated with olfaction
 Lidless lateral eyes with a spiracle behind each
 5-7 gill slits anterior to each pectoral fin
 Tough leathery skin covered in toothlike dermal
placoid scales to reduce turbulence of water
 Jaws with sharp teeth. The front row of functional
teeth on the edge of the jaw is backed by rows of
developing teeth that replace worn teeth
 Mouth cavity opens into a large pharynx which
contains openings to gill slits and spiracles. A short, wide esophagus runs to the j-shaped stomach. A liver and
pancreas discharge into a short, straight intestine. Attached to a short rectum is a rectal gland which secretes a
colourless fluid containing sodium chloride
 Chambers of the heart are arranged in tandem formation.
 Internal fertilisation. Large yolky eggs are laid

Subclass Holocephali: Chimaeras Eg. Ratfishes
 Jaws bear large flat plates
 Upper jaw fused to cranium