Flatworms-In the life cycle of trematode flukes of the subclass Digenea, mollusks (mostly snails) serve as the intermediate host. Fertilized eggs usually hatch in water. The first larval stage, the miracidium, generally is free-swimming and penetrates a freshwater or marine snail, unless it has already been ingested by one. Roundworms-Roundworms can complete their life cycle in immature dogs, but as the pup's immune system matures (usually by 6 months of age), the larval stages of the roundworm will become arrested and will encyst (become enclosed in a cyst) in the pup's muscles. They can remain encysted in the dog's tissues for months or years.
The main groups of unicellular organisms are bacteria<span>, </span>archaea<span>, </span>protozoa<span>, unicellular </span>algae<span>, and unicellular </span>fungi<span>. </span>
