Give the characteristic features of the following,citing one example of each:
$(a)$ Chondrichthyes and Osteichthyes
$(b)$ Urochordata and Cephalochordata

(N/A) Chondrichthyes and Osteichthyes:
Chondrichthyes:
- They are marine animals with a streamlined body and cartilaginous endoskeleton.
- The mouth is located ventrally.
- The notochord is persistent throughout life.
- Gill slits are separate and without an operculum (gill cover).
- The skin is tough,containing minute placoid scales.
- Teeth are modified placoid scales that are backwardly directed.
- Their jaws are very powerful.
- These animals are predaceous.
- Due to the absence of an air bladder,they have to swim constantly to avoid sinking.
- The heart is two-chambered (one auricle and one ventricle).
- Some of them have electric organs (e.g.,$Torpedo$) and some possess a poison sting (e.g.,$Trygon$).
- They are cold-blooded (poikilothermous) animals.
- Sexes are separate. In males,pelvic fins bear claspers.
- They have internal fertilization and many are viviparous.
- Examples: $Scoliodon$ (Dog fish),$Pristis$ (Saw fish).
Osteichthyes:
- They include both marine and freshwater fishes with a bony endoskeleton.
- The body is streamlined. Mouth is mostly terminal.
- They have four pairs of gills which are covered by an operculum on each side.
- Skin is covered with cycloid/ctenoid scales.
- Air bladder is present which regulates buoyancy.
- Heart is two-chambered.
- They are cold-blooded animals.
- Sexes are separate. Fertilization is usually external. They are mostly oviparous and development is direct.
- Examples: $Hippocampus$ (Sea horse),$Labeo$ (Rohu).
$(b)$ Urochordata and Cephalochordata:
Urochordata (Tunicata):
- Notochord is present only in the larval tail.
- Examples: $Ascidia$,$Salpa$,$Doliolum$.
Cephalochordata:
- Notochord extends from head to tail region and is persistent throughout their life.
- Examples: $Branchiostoma$ (Amphioxus or Lancelet).