Answer:
Characteristic
features of different classes mentioned are as follows
(a) Chondrichthyes
(i)These are marine
animals with a stream lined body.
(ii) Body is
has cartilaginous exoskeleton and the skin is tough due to the presence of
minute placid scales.
(iii) These
animals are predaceous.
(iv) Due to
the absence of air bladder they have to swim constantly to avoid spiking
(v) They are
cold blooded animals e.g., Scoliodon (dog fish), Pristis (saw flash)
(vi) Some of
them possess electric organs and poison sting (Torpedo and frygon).
Osteichthyes
(i) These
animals are found in both habitats, i.e., marine and freshwater. They bearbony
endoskeleton.
(ii) Body is
streamlined, possesses four pairs of gills covered by an operculum, on each
side.
(iii) Skin
is covered by cycloid or ctenoid scales.
(iv) Presence
of air bladder is an additional feature providing buoyancy to animal.
(v) Heart is
two chambered.
(vi) Sexes
are separate, fertilization usually external, oviparous animal with direct development.
e.g.,
marine- Exocoetus (flying fish), Hippocampus (sea horse); freshwater- Laoeo
(rohu).
(b)
Urochordata
(i)In urochordates,
notochord is only present in larval tail.
(ii) They
contain a tough outer covering called tunic.
(iii) Presence
of pharyngeal slits during some stage of the life cycle and a muscular, post
anal tail is the characteristic features in urochordates.
e.g., Salpa,
Ascidia, Doliolum.
Cephalochordate
(i) Cephalochordates
are exclusively marine animals.
(ii)The
notochord in cephalochordates extends, from the heart to tail region and is persistent
throughout the life.
(iii) Body-wall
shows one cell thick, non-ciliated epidermis, connective tissue, stated muscle
and parietal peritonium.
(iv) Fertilisation
is external e.g., Branchiostoma (Amphioxus or Lancelot).
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