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The following points highlight the two types of germinal layers of the animals. The types are: 1. Segmentation 2. Development.
Germinal Layers: Type # 1. Segmentation:
Segmentation is division or differentiation of the body into distinct portions called segments. It is of two types: metameric segmentation and pseudo-metamerism.
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I. Metameric Segmentation (True metamerism or True segmentation):
It is a type of segmentation where external divisions correspond to internal divisions. It occurs in three highly organized phyla — Annelida, Arthropoda and Chordata.
The body is often divided both externally and internally into a number of segments (metameres) e.g., annelids. Segmentation is mostly external in arthropods and mainly internal in man and other chordates (vertebrae, body muscles, some blood vessels and nerves).
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II. Pseudo-Metamerism (False segmentation):
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It is found in tapeworms. The pro-glottides (segments of tapeworms) are budded off from the neck and are not of embryonic origin hence, this segmentation is called pseudo-metamerism (pseudo-segmentation). Pseudo-segmentation as found in tapeworms is external only. The body is not internally divided.
i. Cephalization:
Differentiation of a definite head at the anterior end is termed the cephalization. There is a concentration of sense organs, nervous tissue (brain) and food catching organs at the anterior end.
ii. Appendages:
Projecting parts from the body of an animal that serve in locomotion feeding and other ways are called appendages. They include tentacles of cnidarians, minute setae and Para podia of many annelids, antennae, legs and wings of arthropods, foot of molluscs and fins, legs and wings of vertebrates.
iii. Sexual Dimorphism:
When the male and female animals can be distinguished externally the condition is called sexual dimorphism. The latter is seen in male and female frogs peacock and peahen, lion and lioness and human beings.
iv. Fertilization:
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Fertilization is the union of male and female gametes. In animals, male gametes or sperms are motile. The female gametes or ova are non-motile. When fertilization occurs outside the body of the female, it is called external fertilization, e.g., Starfish Frog. In many cases, fertilization takes place in the genital tract of the female. It is known as internal fertilization, e.g. reptiles, birds, mammals.
If the fusion of male and female gametes of the same parent takes place it is called self-fertilization, e.g. tapeworm. When the fusion of male and female gametes of different parents takes place, it is known as cross fertilization, e.g., all fishes, amphibians, reptiles’ birds and mammals.
Germinal Layers: Type # 2. Development:
Development includes changes that an organism undergoes from its beginning to maturity. It is of two types, direct and indirect. In direct development, the young ones resemble the adults in all respects except colour size. There is no intermediate stage in direct development, e.g. Hydra, earthworm and silver fish (insect).
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In indirect development, the young ones do not resemble the adults. The young ones usually pass through one or more intermediate stages before obtaining the shape of the adults. The phenomenon of passing through different juvenile stages before attaining the form of adult is known as metamorphosis, e.g. silk moth.
