Final answer:
The anterior-most portion of the annelid body is known as the prostomium, serving as the 'head' of the organism, whereas the posterior-most part is referred to as the pygidium, containing the anus. Answer 'a. Prostomium' correctly fills the blank in the given statement.
Step-by-step explanation:
The anterior-most and posterior-most portions of the annelid body are the prostomium and pygidium, respectively. The correct answer to fill in the blank is 'a. Prostomium'. The prostomium is essentially the 'head' or the front segment that contains sensory organs, while the pygidium is the final segment that contains the anus. Annelids are organisms with bilateral symmetry and a segmental body structure called metamerism. This segmentation allows for efficient movement and growth of the organism through the addition of body compartments or segments.
Annelids possess a true coelom, allowing their internal organs to be bathed in coelomic fluid. They exhibit teloblastic growth, where new segments are added between the existing body and the pygidium during development. Moreover, annelids have a well-developed nervous system, a closed circulatory system, and excretory structures called metanephridia in each segment.