Ostracods are crustacea that have two symmetrical shells that cover its body. The shrimp like critter lives inside. The shell is composed of calcium carbonate.
Gastrotrichs are wormlike animals with lobed heads that are bilaterally symmetrical. The have adhesive tubes mainly near its posterior that help it cling to surfaces. Their bodies are not segmented and are flat. Their head lobe has small bristles on an oral hood.
Nematodes are cylindrical and slender with rounded ends, which attributes to this group being referred to as roundworms. They lack segmentation and cilia. Nematodes can be found in most habitats including soil, sand, salt flats, ocean, hot springs, and fresh water. There are also parasitic forms that live inside other organisms.
Flatworms (Platyhelminthes) are ribbon-shaped and are adapted to many habitats including land, marine, and fresh water. They are noted for their extensive regeneration. They are not as complex as the true worms (annelids). They have an opening into the gut that acts as food in and waste out portal. Planaria are easily recognized because their head region has two eyes that appear “cross-eyed.” They are free living and eat decaying meat. They have a simple nervous system. Their excretory system consists of specialized “flame cells,” that remove waste.
Rotifers are named for the cilia at the crow of their heads. They are bilaterally symmetrical and covered with an external layer of chitin called a lorica. Rotifers lack a circulatory system and respire through the surface of their body. Most are free swimming individuals although some create colonies that can be seen rotating rapidly like a spinning ball. They feed on bacteria, protists, other rotifers and small animals, and suspended organic matter. Rotifers are a major source of food for other animals in freshwater environments.