Class-Cyclostomata is comprised of, the living jawless fishes. Their mouth is circular and lack jaws, hence they are also called agnathans. It works like a sucker and is surrounded by tentacles (e.g., lampreys and haglish). These also prosses rectroctable horny teeth.

Fishes possess dermal scales, each scale made of dentine that is secreted by dermal papilla. Placoid scales are hard and microscopic in size, their body is made up of dentine and exposed surface is covered with enamel, Cycloid scales are strong and extremely flexible.

They are large, oval and made up of isopectine (layer of collagenous fibre). Placoid Scales are the characteristic features of cartilaginous fishes. (e.g., Scoliodon). Whereas, cycloid scale are found in bony fishes (e.g., Catla catla).

There are certain characters acquired by reptiles for the terrestrial adaptations. Two of then are

(i) Their body is covered by dry and cornified skin and epidermal scales or scutes.

(ii) Fertilisation is internal.

Every living form maintain a proper shape, size and structure. In multicellular lower organisms the shape of body is maintained by certain biological materials like chitin and calcium substances. Chitinous exoskeleton is the characteristic feature of arthropods. (e.g., cockroach, termite,wasp) whereas, calcareous shell is present in molluscs (gastropods). (e.g., snail and slugs).

The phenomenon of production and emission of light by an organism as a result of chemical reaction during which chemical energy is converted to light energy is called bioluminescence.

Ctenoplana from phylum-Ctenophora, exhibits the phenomenon of bioluminescence.