There are three morphologically distinct stages in the life cycle; the egg “finally” develops into larva which is converted into motionless pupa that finally develops into an adult. It some primitive insects the metamorphosis is incomplete.
We often hear that objects appear to be weightless in a spaceship circling round the Earth. In order to examine the effect in some detail, let us first define, what do we mean by the weight? The real weight of an object is the gravitational pull of the Earth on the object.
The body is segmented. Each segment is attached to its neighbor by means of a modified portion of cuticle which is thin and flexible, they possess jointed appendages. These appendages have been modified for specialized functions.
The body is metamerically segmented. The body becomes divided transversely into a number of similar parts or segments. The subdivisions may be indicated externally by constrictions of the body surface.
The name nematode means “pointed ends”. The animals included in this group have elongated worm like body with pointed ends. The nematodes are triploblastic and pseudocoelomates. One end of the body is anterior, however the head is not clearly marked and there are no special sense organs at this end
The platyhelminthes are triploblastic acoelomates. There is development f a third layer, the mesoderm, which separates the ectoderm and endoderm. The playhelminthes exhibit bilateral symmetry, and body is unsegmented.
For example, in Obelia there are feeding individuals, the gastrozooids; the individuals capable of asexual reproduction only, the gonozooids, blastostyles and free-living or sexually reproducing individuals, the medusa…..
Cnidarians have double layer organization and are therefore diploblastic having tissue grade organization and have organs. During the development two germinal layers are formed the outer ectoderm and inner endoderm from which their bodies are constructed.
These animals are composed of many cells however there is no tissue organization and have no organs. Sponges lack symmetry. In most sponges the body wall is formed of an outer layer, pinacoderm, made up of cells called pinacocytes:
Diploblastic animals belong to division radiata. The body of these animals consists of two layers of cells, ectoderm and endoderm. There is a jelly like mesenchyme or mesogloea which in most cases is non cellular. Diploblastic animals show lesser degree of specialization and they do not form specialized organs.
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Two Table are Design and Datasheet View
21 Jan, 2014
