All free-living flatworms are predators that actively hunt for food. Marine species live buried in the sand or under rocks in shallow water. Free-living non-parasitic flatworms are typically less than 10 centimeters long. Parasitic flatworms that live on or inside other animals-including humans-can injure or even kill the host organism. Flatworms live on land, in fresh water, in the ocean, and in or on other animals as parasites (e.g., tapeworms). The name Platyhelminthes (pronounced “plat-ee-hel-MIN-theze”) is derived from the Greek root word platy meaning flat and the Greek root word helminth meaning worm. The phylum Platyhelminthes consists of simple worm-like animals called flatworms (Fig. a circulatory system consisting of a series of tubes (vessels) filled with fluid (blood) to transport dissolved nutrients, oxygen, and waste products around the body rapidly and efficiently.a coelom, a body cavity between the digestive tube and the external body wall that is lined with tissue.a complete digestive system, from an anterior mouth to a posterior anus.

an excretory system to eliminate some kinds of waste products.

a central nervous system guided by a “brain”.

a mesoderm, an intermediate body layer between the inner (endoderm) and outer (ectoderm) tissue layers that forms muscle tissue.

There are six features and systems that reveal an evolving complexity in the body structure of most worms: They can detect the kinds of environment they encounter by moving in the anterior direction. Organs for sensing light, touch, and smell are concentrated in the heads of worms. Figure 3.35 compares bilateral symmetry in a whale shark and a swimming plychaete worm. The lateral surfaces are found on the left and right sides of the body. The dorsal surface is located on the upper part of the body facing the sky. The ventral surface of worms and other organisms is the bottom side of the body, often closest to the ground. Worms have a definite anterior (head) end and a posterior (tail) end. Worms are invertebrate animals with bilateral symmetry. Image courtesy of Uwe Kils, Wikimedia Commons

Steam workshop age of wonders 3