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' flat animals ') [3] is a phylum of free-living (non-parasitic) marine invertebrates. [4] [5] They are blob-like animals composed of aggregations of cells. Moving in water by ciliary motion, eating food by engulfment, reproducing by fission or budding, placozoans are described as "the simplest animals on Earth."
As a result, the animals absorb liquid, begin to swell, and separate from the substrate so that they float freely in the water. In the protected interior space, the ventral cells form an ovum surrounded by a special envelope, the fertilisation membrane; the ovum is supplied with nutrients by the surrounding syncytium, allowing energy-rich yolk ...
The cause of this posture—often called a "death pose"—has been a matter of scientific debate. Traditional explanations ranged from strong ligaments in the animal's neck desiccating and contracting to draw the body into the pose, [1] to water currents arranging the remains in the position. [2]
The word "protozoa" (singular protozoon) was coined in 1818 by zoologist Georg August Goldfuss (=Goldfuß), as the Greek equivalent of the German Urthiere, meaning "primitive, or original animals" (ur-'proto-' + Thier 'animal'). [11] Goldfuss created Protozoa as a class containing what he believed to be the simplest animals. [3]
Cnidarians are the simplest animals in which the cells are organised into tissues. [33] The starlet sea anemone is used as a model organism in research. [ 34 ] It is easy to care for in the laboratory and a protocol has been developed which can yield large numbers of embryos on a daily basis. [ 35 ]
The first known mass extinction was the Great Oxidation Event 2.4 billion years ago, which killed most of the planet's obligate anaerobes. Researchers have identified five other major extinction events in Earth's history, with estimated losses below: [ 11 ]
Not much is known about the lifecycle of C. exigua. It exhibits sexual reproduction. The species starts as a juvenile in a short, free-living stage in the water column. [2] Juveniles likely first attach to the gills of a fish and become males. As they mature, they become females, with mating likely occurring on the gills.
In the proboscisless leeches, the jaws (if any) of Arhynchobdellids are at the front of the mouth, and have three blades set at an angle to each other. In feeding, these slice their way through the skin of the host, leaving a Y-shaped incision. Behind the blades is the mouth, located ventrally at the anterior end of the body.