Question

Animals having cylindrical body having an organ system level of organization showing metamerism belongs to phylum (a) Arthropoda (b) Mollusca (c) Annelida (d) Platyhelminthes

Short answer

The correct answer is (c) Annelida.

Step-by-step solution

Analyze the options

List the characteristics of each phylum mentioned in the options and analyze each of them in relation to the characteristics provided in the question.

Identify the phylum with a cylindrical body

Three of the given phyla have animals with a cylindrical body: Arthropoda, Annelida, and Platyhelminthes. However, Mollusca is excluded as it does not typically have a cylindrical body.

Identify the phylum with organ-system level of organization

All of the mentioned phyla have organ-system level of organization. Therefore, this characteristic does not serve as a differentiating factor.

Identify the phylum with metamerism

Among the options, Annelida is the only phylum known for showing metamerism, where the body is segmented both internally and externally.

Conclude the answer

After going through each characteristic, the only animal phylum that fits all the provided traits (cylindrical body, organ-system level of organization, and metamerism) is Annelida. Therefore, the answer is (c) Annelida.

Key concepts

Metamerism in Animals

Metamerism is a fascinating biological concept observed in some animal phyla, most notably within the Annelida. It refers to the segmentation of the body into a series of repetitive parts called metameres or segments. This structural organization is evident both internally and externally, with each segment housing a repetition of various organs. For example, earthworms, which are annelids, exhibit this characteristic with multiple ring-like segments throughout their body.

In the context of the textbook exercise, metamerism is a key identifying trait for the phylum Annelida. This type of body structure is advantageous for the animals it characterizes; it allows for greater flexibility and mobility. Moreover, if one segment is damaged, others can often continue functioning without a fatal impact on the organism. In contrast to Annelids, other mentioned phyla such as Arthropoda or Platyhelminthes show different forms of body segmentation and organization, not conforming to true metamerism.

Organ System Level of Organization

The organ-system level of organization is a complex layer of biological organization where organs work together within systems to perform specific functions necessary for an organism's survival. This level of organization is evident in most multicellular animals, including all the phyla mentioned in the exercise question: Arthropoda, Mollusca, Annelida, and Platyhelminthes.

Each organ system in these phyla, such as the digestive or circulatory system, has a specialized role. This collaborative approach at the organ-system level offers increased efficiency and higher levels of specialization. For instance, annelids possess a well-developed digestive system that extends through the length of their body, and a circulatory system that is closed, meaning that their blood flows within vessels throughout each segment. The presence of this level of complexity in Annelida points to their evolutionary advancement as compared to simpler forms of life where organ systems are not present or less developed.

Cylindrical Body in Zoology

A cylindrical body shape is found in various animal phyla but is quite distinctive within the phylum Annelida. This zoological characteristic refers to the body being longer than it is wide, with a consistent diameter throughout. Such a body plan is conducive to burrowing and moving through substrates, as seen in the lifestyle of many annelids like earthworms and polychaetes.

In Annelida, the cylindrical body is not only practical for their environmental interactions but also works in tandem with their metamerism. The flexible, elongated shape allows for the efficient contraction and expansion of segments, enabling subtle and precise movements. This cylindrical design of Annelids provides them with a hydrostatic skeleton, in which fluid within the body cavity can change distribution, allowing fluid movements and resistance against external pressure. Contrast this with the varied body shapes of Arthropoda and Platyhelminthes, and we can appreciate how a cylindrical form is suited to the lifeways of segmented worms within the phylum Annelida.