While a biology student goes to the beach, he happens to see crabs and starfishes on the sea shore.

Explanation

Crab:
1. Crab belong to phylum arthropoda.
2. (G. arthros- jointed, podes- feet) This is the largest phylum of the Kingdom Animalia and includes the largest class called Insecta (total species ranges from 2-10 million).
4. They are bilaterally symmetrical, segmented, and triploblastic and schizocoelomate animals with organ system grade of body organisation.
They have jointed appendages which are used for locomotion, feeding and are sensory in function.
5. Body is covered by chitinous exoskeleton for protection and to prevent water loss, It is shed off periodically by a process called moulting or ecdysis.
6. The body consists of a head, thorax, and abdomen with a body cavity called haemocoel.
7. Respiratory organs are gills, book gills, book lungs or trachea.
8. Circulatory system is of open type. Sensory organs like antennae, eyes (compound and simple), statocysts (organs of balance/equilibrium) are present.
9. Excretion takes place through malpighian tubules, green glands, coxal glands, etc.
10. They are mostly dioecious and oviparous, fertilization is usually internal.
11. Development may be direct or indirect.
12. Life history includes many larval stages followed by metamorphosis.
Starfishes:
1. Star fish þelong to phylum Echinodermata. (G. Echinos = spiny, dermos -skin).
2. All Echinoderms are marine animals.
3. The adults are radially symmetrical but the larvae are bilaterally symmetrical.
4. These animals have a mesodermal endoskeleton of calcareous ossicles and hence the name Echinodermata (spiny skin).
5. They are exclusively marine with organ system level of organisation.
6. The most distinctive feature of echinoderms is the presence of the water vascular system or ambulacral system with tube feet or podia, which helps in locomotion, capture and transport of food and respiration.
7. The digestive system is complete with mouth on ventral side and anus on the dorsal side.
8. Excretory organs are absent.
9. The nervous system and sensory organs are poorly developed.
10. The circulatory system is open type without heart and blood vessels.
11. Sexes are separate. Reproduction is sexual and fertilization is external.
12. Development is indirect with free swimming bilaterally symmetrical larval forms.
13. Some echinoderms exhibit autotomy with remarkable powers of regeneration.