Question

How are annelid worms similar to members of phylum platyhelminthes, nemertean, gnathostomulida, nematode?

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Answer 1

Worm classification begins from the phylum Platyhelminths, also known as the flatworms. Flatworms have no segmentation, absence of coelom (body cavity), an incomplete digestive system, and a total absence of a specialized circulatory and respiratory system (oxygen and nutrients pass the body via diffusion). They have specialized cells called flame cells that perform the function of excretion and osmoregulation. These worms are distinguished by their distinct flat bodies and the presence of suckers. They are hermaphroditic worms, and are mostly endoparasitic in nature.

Nemertea is a phylum very similar to platyheminths, as the members of this phyla are also acoelomates and depend on flame cells for osmoregulation and excretion. In comparison to flatworms, their digestive systems are well developed. The most disitnct feature of these worms is the presence of a proboscis in a cavity called rhynchocoel, that helps in capturing the prey by spraying venom. These worms are also known as Rhynchocoela.

Gnathostomulids are marine jaw worms that live in environments with low oxygen levels. Just like the above two phyla, these worms have a ciliated epidermis, are acoelomates and lack a proper circulatory and respiratory system. Their digestive systems are not as developed as the Nemertia.

Nematodes, also known as roundworms, are pseudo-coelomates, with a complete digestive system and elongated bodies. They lack a circulatory and respiratory system and are dioecious in nature. These worms are capable of living in a wide range of environments.

Annelids, also known as ringed or segmented worms, have a true body cavity (coelomates), are segmented, and have a complete digestive system. They have a functioning circulatory and respiratory system, unlike the above mentioned phyla.

The above paragraphs show the level of development and salient feature possessed by the different phylum consisting of worms. It also shows how some features remain the same in most phyla and that leads to the following points:

Annelids are similar to the members of phyla Platyhelminthes, Nemertea, Gnathostomulida, and Nematodes in the following way -

• The members are all these phyla have an 'organ & organ system' level of organization.
• All the members show bilateral symmetry.
• Digestion takes place in a form in all the members, however the digestive system is complete only in Annelids and Nematods and the other phyla have an incomplete digestive system.
• All the members have a ciliated epidermis, though the number of cilia per cell vary.
• All the members prefer a moist environment.
• All the members have an elongated worm-like body and in most cases is slimy.
• Most of the members of the mentioned phyla are parasitic in nature.