FURTHER PHYLOGENETIC CONSIDERATIONS There are different views about the evolution of platyhelminthes: Zoologists believe that the platyhelminthes body form is central to animal evolution. They believe that an ancestral flatworm…
PHYLUM GASTROTRICHA (Or. gastros, stomach i- trichos, hair) They have about live hundred free- living marine and freshwater species. They live in the space between bottom sediments. Their size is…
PHYLUM NEMERTEA There are nine hundred species of nemerteans. They are elongate, flattened worms. They are found in marine mud and sand. They have a long proboscis. Therefore, nemerteans are…
Evolution of the vertebrate nervous system included development of a single. tubular nerve cord above the notochord. During early evolution, the nerve cord underwent expansion, regional modification, and specialization into…
Although no real nervous system is present in protozoa and sponges, coordination and reaction to external and internal stimuli such as, to light, temperature, chemicals etc do occur Only animals…
When an action potential passes down an axon to its terminal, it must cross a small gap, the synapse (Gr. synapsis, contact, union), separating it from another neuron or an…
Saltatory Conduction is different means of speeding the propagation of action potentials has evolved in vertebrates. Many axons in vertebrate nervous systems are myelinated, that is, coated with insulating layers…
CLASS CESTOIDEA: THE TAPEWORMS It is the most highly specialized class of flatworms. The members of cestoidae are commonly called tapeworms. It has 3,500 species. All are endoparasites. They live…
Stages / Phases of Action Potential: 1. All cells have a membrane potential; however, only certain kinds of cells, including neurons and muscle cells, have the ability to generate changes…
CLASS TREMATODA: Flukes (trematodes, perforated form) This class has eight thousand species of parasitic flatworms. Trematoda are collectively called flukes. They are wide and flat shaped. 1. Habitat: Almost all…
saadia
rubab