The characteristic feature of Ascaris is

The correct option is pseudocoelom and no metamerism.

To understand why this is the characteristic feature of Ascaris, let us break down the terms and analyze the organism's biological classification:

1. Pseudocoelom (False Coelom):
Ascaris belongs to the phylum Aschelminthes (commonly known as roundworms or nematodes). In these organisms, the body cavity is not lined by mesoderm. Instead, the mesoderm is present as scattered pouches between the ectoderm and endoderm. Such a body cavity is called a pseudocoelom, and the animals possessing it are called pseudocoelomates.

2. No Metamerism (Lack of Segmentation):
Metamerism is the phenomenon where the body is externally and internally divided into repetitive segments (metameres), as seen in annelids (like earthworms) and arthropods. Ascaris has an unsegmented, cylindrical body covered by a tough, resistant cuticle. Since it lacks true segmentation, it shows no metamerism.

Therefore, the combination of having a pseudocoelom and showing no metamerism is the defining characteristic feature of Ascaris.