(a) Snails and (b) slugs are both gastropods, but slugs lack a shell. The intestine is present in the form of diverticular pouches and ends in a rectum that opens via an anus. When the muscle tissue is eaten by the primary host, the cycle is completed. In many dioecious species, males are short-lived and smaller with no digestive system and a single testis.

Nemerteans have almost no predators and two species are sold as fish bait. These animals have well-developed neuronal and digestive systems. However, cross-fertilization is preferred in hermaphroditic animals. Phylum Mollusca is a very diverse (85,000 species) group of mostly marine species. Watch the video below to see a nemertean attack a polychaete with its proboscis.
Gastropoda includes shell-bearing species as well as species with a reduced shell. There are five phyla in the superphylum Lophotrochozoa (also known as the crest or wheel animals): Platyhelminthes, Rotifera, Nemertea, Mollusca, and Annelida. Embryonic development of nemertine worms proceeds via a planuliform larval stage.

The head is rudimentary and protrudes out of the posterior end of the shell. Many of the eggs are released in feces and find their way into a waterway, where they are able to reinfect the primary snail host. Trematodes are responsible for serious human diseases including schistosomiasis, a blood fluke.

Figure 5. (c) The Trematoda class includes Fascioloides magna (right) and Fasciaola hepatica (two specimens of left, also known as the common liver fluke). and Hirudinea (leeches). Reproductive strategies include sexual dimorphism, hermaphroditism, and serial hermaphroditism.

Some mollusks have evolved a reduced shell. The rotifers are filter feeders that will eat dead material, algae, and other microscopic living organisms, and are therefore very important components of aquatic food webs. The Platyhelminthes consist of two lineages: the Catenulida and the Rhabditophora. The visceral mass is present above the foot, in the visceral hump. Larger body size means more organ systems. Eggs and sperm are released into the water, and fertilization occurs externally. (a) Species from the class Bdelloidea are characterized by a large corona, shown separately from the whole animals in the center of this scanning electron micrograph. These animals do not possess eyes, but they have a radula, as well as a foot modified into tentacles with a bulbous end, known as captaculae. Annelids display bilateral symmetry and are worm-like in overall morphology. Cestodes, or tapeworms, infect the digestive systems of primary vertebrate hosts. Mollusks are predominantly a marine group of animals; however, they are known to inhabit freshwater as well as terrestrial habitats. Most monogeneans are hermaphroditic, but the male gametes develop first and so cross-fertilization is quite common. As the name suggests, bivalves are enclosed in a pair of shells (valves are commonly called “shells”) that are hinged at the dorsal end by shell ligaments as well as shell teeth (Figure 10).