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The acoelomate animals include these three phyla: the flatworms (Platyhelminthes), ribbon worms, and jaw worms. These are primitive organisms but have bilateral_symmetry. They have three germ_layers and are at the organ-system level of organization. They are protostomes and have spiral_cleavage. They exhibit cephalization. Their excretory mechanism includes the flame_cell. They have simple sense organs such as eyespots. Their nervous system consists of sensory ganglia and a longitudinal nerve. They have mucus-producing structures known as rhabdites in their epidermis.

Class_Turbellaria:

These are the "free-living flatworms", a/k/a planarians. These organisms are hermaphrodites. They move by gliding with help of cilia. They feed by projecting a pharynx from their mouth and they are carnivores. Gas exchange and some waste removal is accomplished by diffusion. Both asexual and sexual reproductive mechanisms are utilized. These organisms can also reproduce by regeneration.

Class_Monogenea:

Parasites known as flukes, a/k/a the trematodes. They have a protective body covering called the tegument. Two types of flukes affect humans: blood_flukes (Schistosoma) and liver_flukes (Clonorchis).

Class_Cestoda:

Parasites called tapeworms. They have a distinctive head (cephalic) region called the scolex, and body segments known as proglottids.

Some Cestode scolices (sing: scolex) have hooks, some have suckers and some have both. The scolex is used to attach the new tapeworm to the intestinal wall.

The genital_pore is located on one of the two exterior sides of each proglottid_segment. The uterus is centrally situated and in line with the vagina and genital pore. Testes are located above the uterus and the ovaries are located below the uterus. The yolk_glands are located along the bottom interior edge of each proglottid.