Acoelomates: Phylum Platyhelminthes and Nemertea - Biosciweb.net

Acoelomates: Phylum Platyhelminthes and Nemertea and
Pseudocoelomates: Phyla Nematoda and Rotifera & Parasitism 6.2
Lab #6 - Biological Sciences 102 – Animal Biology
Classification & Adaptations of Acoelomate Animals
Phylum Platyhelminthes (flatworms)
Class Turbellaria (tur'bel-lar' e-a) (L. turbellae, stir, bustle, + aria, like or connected with).
About 3000 species; Turbellarians. Mostly free-living, with a ciliated epidermis. A paraphyletic
grouping. Example: Dugesia tigrina
Class Monogenea (mon'o-gen'e-a) (Gr. mono, single, + gene, origin, birth). About 1100 species;
Monogenetic flukes. Adult body covered with syncytial tegument without cilia; leaflike to
cylindrical in shape; posterior attachment organ with hooks, suckers, or clamps, usually in
combination; all parasitic, mostly on skin or gills of fishes; single host; monoecious; usually
free-swimming ciliated larva. Examples: Polystoma, Gyrodactylus
Class Trematoda (trem'a-to'da) (Gr. trematodes, with holes, + eidos, form). About 10,000
species; Digenetic flukes. Adult body covered with nonciliated syncytial tegument; leaf-like or
cylindrical in shape; usually with oral and ventral suckers, no hooks; development indirect,
first host a mollusc, final host usually a vertebrate; parasitic in all classes of vertebrates.
Examples: Fasciola, Clonorchis, Schistosoma
Class Cestoda (ses-to'da) (Gr. kestos, girdle, + eidos, form). About 3500 species; Tapeworms.
Adult body covered with non-ciliated, syncytial tegument; scolex with suckers or hooks,
sometimes both, for attachment; long, ribbonlike body, usually divided into series of
proglottids; no digestive organs; parasitic in digestive tract of all classes of vertebrates; first
host may be invertebrate or vertebrate. Examples: Taenia, Diphyllobothrium
Phylum Nemertea or Rhynchocoela (ribbonworms)
About 600 species. Ribbon worms are often called nemertine or nemertean worms. Nearly all
are marine and are characterized by an eversible proboscis that can be thrown out with great
speed to capture food. Ribbon worms have separated the two functions of the gastrovascular
cavity in flatworms and have a complete mouth-to-anus digestive tract and a circulatory
system. Nemerteans occur in the intertidal, under stones or sea weed, or in burrows in muddy
sand. Nemerteans can be very long at up to 50 meters (these species are the worlds
longest animals).
Body Plan Features & Characteristics of Members of the Phylum Nemertea
(some of these traits are seen in other animal phyla in addition to the Phylum Nemertea):
1. acoelomate, bilateral symmetry, triploblastic structure
2. some degree of cephalization = head with sensory organs
3. some organ systems (formed of true tissues) are present
4. eversible proboscis (only worm phylum with this characteristic)
5. complete digestive tract/system (from mouth-to-anus)
6. separate circulatory system with two lateral vessels on each side of the gut and
sinuses around head and tail; no definite closed circulation
7. flame cells for excretion
8. longitudinal and circular muscles contract against fibrous basement membrane;
multiple patterns of muscle fiber organization
9. hydrostatic skeleton (as muscles push against parenchyma and tissue fluids)
10. dioecious with sexual reproduction or asexual fragmentation
11. not parasitic, free-living