Even though the article was longer than I intended, I still covered so little of the available information on cnidarians. Most people probably do not know much about either group, and I would hazard to guess that the majority of even reef-keepers have not noticed these animals in their aquariums. They display a wide variety of body forms and are successful inhabitants of an even greater range of environments. Other turbellarians are terrestrial and some are even symbiotic in or on other invertebrates. I will only briefly mention the former three classes in this article, because these groups are generally only of concern to hobbyists as pathogens, and their biology and classification is probably not of general interest.
Some terrestrial forms are very elongate, and appear more ribbon-like than oval.
Some species have tentacles on the anterior end, and are occasionally mistaken for nudibranchs by the untrained observer. The free-living forms generally range from less than 1 mm to over 30 cm in length, but the largest flatworms are tapeworms, some of which can attain lengths of several meters. Unlike the previous groups we have discussed, the flatworms have three tissue layers, similar to the rest of the groups we will discuss, right through vertebrates. A terrestrial flatworm, photographed in Seattle, Washington. These animals control their osmotic water balance with specialized structures called protonephridia , which function in much the same way as our own kidneys.