Phylum Platyhelminthes
Defining Characteristics
“Acoelomate” refers to type of body cavity
“Triploblastic” refers to derivation of all adult organs & tissues from 3 embryonic layers.
Platyhelminths are endowed with protonephridia for excretion of metabolic wastes.
The vast majority of flatworm species are simultaneous hermaphrodites
There are 5 classes of Platyhelminthes
Class Turbellaria
A typical example is Dugesia sp.
PPT Slide
Class Cestoda (tapeworms)
The scolex is a holdfast organ with which the tapeworm resists the force of peristaltic constractions. They will have adhesive suckers, and some have spines as well.
Proglottids, or segments, grow in series form the scolex. Proglottids closest to the scolex are immature. Those “in the middle” show complete male & female reproductive systems. Those that are distal to the scolex are gravid.
Most cestodes are quite long, occupying the entire small intestine of its host, which in humans, can be 25-30 feet in length.(!)
I don’t care what you THINK this LOOKS like, it is the scolex of Bothriocephalus. Any questions?!
Echinococcus granulosus, a tapeworm of dogs, is usually ~ 1 cm long, consisting of a scolex followed by 1 immature, 1 mature, and 1 gravid proglottid.
Larvae of cestodes reside in prey of definitive host. The larvae of H. diminuta, the rat tapeworm, reside in insects.
Class Monogenea
Gyrodactylus sp. is a parasite of freshwater fish.
Adults are found on skin of fish
Class Trematoda, Subclass Digenea (Flukes)
Polyembryony is a form of cloning of intermediate stages.
Miracidium hatches out of egg and must find a snail host.
One miracidium enters, but thousands of cercariae may emerge several weeks later.
Most digenetic trematodes are hermaphroditic, as in Clonorchis sinensis, the Chinese liver fluke.
Class Trematoda, Subclass Aspidogastrea
Email: seisen
Home Page: http://www.cbu.edu/~seisen/
Other information: Review of Platyhelminthes