Mollusks are mostly dioecious and are divided into seven classes. The phylum Annelida includes worm-like, segmented animals. Segmentation is both external and internal, which is called metamerism. Annelids are protostomes. Annelids are segmented worms, such as earthworms, in Phylum Annelida. There are about 15,000 species of annelids. They range in length from less than a millimeter to more than 3 meters.     Annelids are divided into many repeating segments. Segmentation of annelids is highly adaptive. Each segment has its own nerve and muscle tissues. This allows the animal to move very... Show more

Mollusks are mostly dioecious and are divided into seven classes. The phylum Annelida includes worm-like, segmented animals. Segmentation is both external and internal, which is called metamerism. Annelids are protostomes.

Annelids are segmented worms, such as earthworms, in Phylum Annelida. There are about 15,000 species of annelids. They range in length from less than a millimeter to more than 3 meters.    
Annelids are divided into many repeating segments. Segmentation of annelids is highly adaptive. Each segment has its own nerve and muscle tissues. This allows the animal to move very efficiently. Some segments can also be specialized to carry out particular functions. They may have special structures on them. For example, they may have tentacles for sensing or feeding, “paddles” for swimming, or suckers for clinging to surfaces.    
Annelids have a large coelom and several organ systems. Their organ systems include a circulatory system, an excretory system, a complete digestive system, and a nervous system. The nervous system includes a brain and sensory organs.    
Most annelids can reproduce both asexually and sexually. Asexual reproduction may occur by budding or fission.    
Sexual reproduction varies by species. Some species go through a larval stage before developing into adults. Other species grow to adult size without going through a larval stage.

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