Aquatic and Marine Biology

Initiation of heart beat may be by extrinsic nerves (neurogenic) as in many adult arthropods, or by an internal pacemaker (myogenic) as in vertebrates.

 Scorpion gives birth to live young after the eggs hatch inside the animal.

 All fishes have two-chambered heart.

 Chondrichthyes include cartilaginous fish.

 Placoid scales are found in cartilaginous fish like sharks and rays.

Starfish have developed water vascular system for locomotion.

Octopus has muscular arms for locomotion.

Gills greatly increase the surface area for gas exchange and they occur in a variety of animal groups including arthropods (including some terrestrial crustaceans), annelids, fish, and amphibians. 

Sharks are cartilaginous fish, meaning their skeletons are made of cartilage rather than bone. They still possess teeth, gills, and livers.

Obelia is a polymorphic colonial marine coelentrate which occurs in shallow coastal waters and is also known as sea fur.

Aplysia, commonly known as the sea hare, is a genus of medium-sized to very large sea slugs, which are marine gastropod mollusks.

Fishes use gills to extract dissolved oxygen from the water, which is their primary method of respiration.

 Amphibians are mesolecithal . 

Option (2) is the correct answer.

Ans - 3