* in some older books these have all been called Bilateria
* the development of bilateral symmetry is one of the most important traits in higher animals
* it implies that a single line or plane will divide the body into equal halves or mirror images
* bilateral symmetry has allowed several other important changes in body structure
* Cephalization- concentration of the nervous tissue into a head; which allow for a single organ to direct the functions of the animal
* allow for greater organ development
* allows for greater differentiation of structure; such as appendages
* Advantages of bilateral symmetry:
* better coordinated movement
* much quicker and more precise response to stimulation; since it is directed by a central nerve center.
* those organisms that do not have a true coelom or body cavity
* are entirely solid except for the gastric cavity or coelenteron
* Turbellaria- free-living
* Monogenea- parasitic, flukes
* Trematoda- parasitic, flukes
* Cestoda- parasitic, tape worms
* bilaterally symmetrical* triploblastic; 3 distinct tissue or germ layers
* dorsoventrally flattened
* lack an anus
* coelom- solid mesenchyme (mesoderm); first phylum to show a definite cellular mesoderm
* have no true respiratory system or circulatory system
* first phylum to show distinct excretory system
* are usually hermaphroditic
* few live in freshwater (ie. Planaria), marine as well as moist terrestrial habitats
* Classified into 2 groups, which is based primarily on egg type, presence or absence of yolk glands, cleavage and level of organization
* Archoophoran Turbellarians
* primative
* yolk glands
* eggs are entolecithal
* cleavage is spiral
* almost all are marine
* Neoophoran Turbellarians* more advanced
* modified spiral cleavage
* advance organization
* eggs ectolecithal
* freshwater and marine and terrestrial
* cilia are usually best developed on the ventral surface and function in locomotion
* epidermis also has a large number of mucous glands that secrete mucous that is used by the cilia in locomotion
* epidermis also has rhabdites who's function is not well understood; thought to function in protection or excretion or both.
* 2 layers of oblique muscles laid in opposite directions
* 1 layer of circular muscles and
* 1 layer of longitudinal muscles
* movement is accomplished by contract of longitudinal muscles first then circular muscles
* these in turn elongate and compress the body
* movement to right or left is done by oblique and bundles of longitudinal
* contraction of muscles is usually peristaltic and waves of contraction can bee seen when they move
* Locomotion is of 3 general types* swimming action of cilia
* swimming or crawling by peristaltic action of muscles
* some terrestrial species have leech-like looping movements
* as are muscles , gonads and excretory structures
* capture of prey is done by wrapping themselves around it and entangling it with mucous
* they ingest the whole prey or suck its juices through a hardened stylet (modified pharynx)
* in many species the pharynx is completely eversible and can envelope the entire prey
* this is the first group that possess a specialized excretory structure
* PROTONEPHIDIUM:
* consists of bulb like structure "flame bulbs" that empty into ducts which empties into a main duct and it empties to the outside via a pore called a nephridiopore
* each bulb consists of fine mesh structure to sieve out solid particles* These are best developed in freshwater forms because they have a big osmoregulatory problem; so their primary function is to get rid of excess water
* most nitrogenous wastes, wastes from digestion are carried through the epidermis
* Like cnidarians turbellarians possess a nerve net, but they also show a beginnings of a well developed central nervous system; so there is a degree of cephalization
* There are a variety of sensory cells and glands; most are chemo- or tactile receptors* there are usually 2 eye spots or ocelli, which can discriminate varying light intensities
* the most complex organ system of turbellarians is the reproductive system
* most are hermaphroditic
* both sex organs discharge gametes into a common chamber even though this is true- cross fertilization is the most common mode of reproduction
* some self fertilization can and does occur
* usually the eggs and sperm are produced at different times in the individual
* the system consists of a pair of ovaries that deposit eggs into an oviduct leading from each; there are a series of vitelline glands that supply food to the eggs
* the system consists of a series of scattered testes that drain sperm into a vasa deferens and into a seminal vesicle; that is connected to a penis bulb that is used as an intromittent organ
* In sexual reproduction there is actual copulation in which the penis bulb of one is inserted into the other, and sperm is deposited
* fertilization is internal
* fertilized eggs are usually deposited in clusters
* in some freshwater worms there are summer and winter eggs
* winter eggs have a hard outer covering that can survive desiccation and freezing
* in freshwater forms eggs hatch into miniature adult forms (ie larval form passed in egg)
* in marine forms (not all) a larva is produced called a Müller's larva which is free swimming
* There is asexual reproduction in turbellarians; it is done by fragmentation and regeneration* many studies have been done on regeneration in planaria