Question

Locomotion occurs in sand worm or Nereis through
A. Chaetae
B. Setae
C. Legs
D. Parapodia

Answer 1

Hint: Nereis or Neanthes is a polychaete marine annelid that lives in sea bottom burrows and comes out to prey on small animals in the night. N. (Sandworm) virens, N. Pelagica (species-type) and N. A common species of Nereis is succinea (Clam worm).

Complete Answer:
- The dorsoventrally flattened, segmented and 30-40 cm long body of Nereis has 80 to 120 segments and can be divided into Acorn, trunk and pygidium.
- On the ventral side, Acron is divisible into the anterior prostomium and posterior peristomium and a mouth in between. Trunk is metamerically segmented, and metameres or somites are called segments, each of which bears a pair of parapodiums.
- The last section is called pygidium, which, on the dorsal side, bears a slight anus and a pair of threads on the dorsal side, such as the anal cirrus, and sensory papillae for several minutes.
- Parapodia are paired body locomotory organs attached to each trunk segment on the lateral side. Each parapodium, viz., is composed of two lobes. Notopodium dorsally and neuropodium ventrally positioned.
- Thread-like appendages, called dorsal and ventral cirri, are carried by both lobes. Two chitinous rods known as aciculum support the two parapodium lobes internally, on the apical portion of which there are bundles of long, chitinous setae or chaetae that project beyond the outer margin.
- Nephridiopore is located near the ventral cirrus, on the parapodium. Nereis can actively crawl, burrow, and swim. Crawling is carried out by parapodia, while swimming involves lateral body undulations, caused by wave-like longitudinal muscle contractions and oar-like use of parapodia.

The correct Answer is option (D) Parapodia.

Note: They are cylindrical in shape, found in sandy regions, and are suitable for burrowing.The skin is made of tall columnar cells and dispersed glandular and sensory cells, and the blood vessels are richly supplied.