Question

The first body segment of an earthworm is,
A. Prostomium
B. Peristomium
C. Peristome
D. Protome

Answer 1

Hint: Earthworms are found to be formed of 100-200 segments called somite’s or metameres, as the annuli are present, which are the ring-like grooves.

Complete step by step answer:
The first body segment is called the periosteum also called buccal segment which contains the mouth. Adult earthworms that are sexually mature, have a swelling called the clitellum which is located about one-third of the way down the earthworm. The clitellum is the glandular collar thickening that surrounds the 14th to 16th segments. The periosteum is the first true body segment in an annelid’s body like an earthworm in the anterior end. It is situated right behind the prostomium. The prostomium and peristomium are variously fused. Since the growth of the annelids occurs from the posterior end, the oldest segment of the annelids is considered to be as the peristomium. Prostomium and peristomium are the two segments of annelid’s body. Annelids are triploblastic animals having a bilateral symmetry. They are the protostomes whose blastopore develops into the mouth. Annelids are segmented worms as well. The body wall, body cavity, nervous system and the kidneys are segmented. The major difference between them is that prostomium is in front of the mouth of annelids while the periosteum surrounds the opening of the mouth. Prostomium contains sensory organs for helping annelids to thrust in the soil.

Note: The similarities of peristomium and prostomium are as follows:
i. The prostomium and peristomium can be variously fused, either completely distinct or comprising a joint structure of a peristomial ring and a tentacular crown.
ii. Both of them are situated in the anterior section of the annelid’s body.