Chapter 35, Problem 11STP

Summary Introduction

To explain:

How the different body structures in roundworms and annelids enable them to move.

Introduction:

Earthworms are annelids that are described by animals with a body plan consisting of segments. There are around 11,000 species of annelids; most of them live in marine water. Annelids include parasitic leeches, earthworms, and marine worms. Annelids are different from roundworms because they are segmented and have a coelom.

Explanation of Solution

When an annelid moves, it binds circular muscles around each segment. This tightens the segment and causes the fluid in the coelom to press outwards. As the fluid in the coelom is restricted to the tissues between the segments, the pressure of the fluid causes the segment to become longer and thinner. After that, the annelid contracts the longitudinal muscles that run the length of the body. This creates the segment to reduce and return to its original shape, pulling forward its posterior end, resulting in movement.

Roundworms do not have bones but they have longitudinal and circular muscles to keep their shape. These muscles are durable and can contract and relax. The muscles also allow them to move and hold their body shape. The movements of roundworms are through the internal part of the host, which is by thrashing their bodies using the muscles and because of this, they can only move sideways.

Conclusion

Thus, the basic features of locomotion in annelids are most easily observed in the earthworm. Movement involves extending the body, anchoring it to a surface with setae, and contracting body muscles.