Human Nervous System

The correct option is (B) : Myelin sheath is discontinuous

Because of the Ranvier nodes, the myelin sheath is disjointed. Nodes of Ranvier, a recurring opening in the myelin sheath that surrounds the axon of certain neurons, help nerve impulses to travel more quickly. A phenomenon called as saltatory conduction allows impulses to move from node to node because nodes of Ranvier periodically interrupt the insulation. The outermost nucleated cytoplasmic layer of Schwann cells, also known as neurolemmocytes, that encircles the neuron's axon is known as the neurolemma.

The question requires identifying the non-chordate characteristics from the given statements. Let's analyze each one:

A. Pharynx is perforated by gill slits: This is a feature commonly found in chordates rather than non-chordates.

B. Notochord is absent: This is true for non-chordates since they lack a notochord.

C. Central nervous system is dorsal: This is characteristic of chordates, not non-chordates.

D. Heart is dorsal if present: Non-chordates usually have a ventral heart if they have one, making 'dorsal heart' unlikely for non-chordates.

E. Post anal tail is absent: Most non-chordates do not have a post-anal tail.

Based on the analysis, the correct statements matching non-chordate characteristics are B (Notochord is absent) and E (Post anal tail is absent). Hence, the correct answer is: B. D & E only