Question

Primitive nervous system is found in
(a) Porifera
(b) Cnidaria
(c) Echinodermata
(d) Annelida

Answer 1

Hint: Nervous systems are made up of numbers of neurons that help to control various body functions. The evolution of the nervous system takes place with the evolution of body structure.

Complete Answer:
Cnidarians are mostly marine, sessile, and free-swimming animals. They are radially symmetrical with tissue level of organization. They are diploblastic animals derived from ectoderm and endoderm. The primitive nervous system consists of a network of nerve cells, nerve cells consist of sensory neurons and motor neurons which carry chemical impulse in the body. Respiration and excretion take place through the body surface by diffusion. The nematocyst is present which is used for defense which is also known as stinging cells. The digestive system is incomplete.

Additional information:
Porifera is an aquatic marine animal. The body of Porifera is porous with Ostia inside and oscula outside. Sponges show the cellular level of the organization. Digestion is intracellular and takes place inside food vacuoles. The exchange of gases and removal of the waste product takes place by diffusion. The nervous system is absent. Skeletons of sponges are made up of calcareous or siliceous spicules.
Echinoderms are marine animals with spines at the body surface. They are a triploblastic animal with an organ system level of organization. The digestive tract is complete with specialized excretory organs. The nervous system consists of nerve rings and radial nerve cords. Sense organ is poorly developed.
Annelida occurs in freshwater, seawater, and moist soil. They are metamerically segmented with a ring-like annulus. They are bilaterally symmetrical triploblastic animals. They have an organ system level of organization. They are coelomate animals with the presence of coelomic fluid. The digestive tract is complete. The nervous system consists of the nerve ring and a solid, double, mid ventral nerve cord with ganglia, and these ganglia are formed by aggregation of nerve cells. The excretory system consists of coiled tubular nephridia. The blood vascular system is of closed type.

So, the correct answer is ‘Cnidaria’.

Note: The cell of Porifera doesn't show a connection with each other, due to the absence of synaptic junction between cells, neurons are not formed inside Porifera. Communication takes place by calcium waves. Porifera is a primitive invertebrate of the animal kingdom but it doesn't possess neuron due to the absence of synaptic connection between cells, in Porifera communication and conduction take place by calcium wave which takes place by body contraction.