Amazing Contents

- Hirudinaria is hermaphrodite but show cross fertilization. - They copulate in water or on land especially during the months of March and April. Two leeches meet in such a way that male genital pore of each is opposite to the female genital pore of the other. Each copulant transfer spermatozoa by the penis into vagina of another leech i.e., they show reciprocal insemination. After about an hour copulants separate.  - In Hirudinaria, fertilization takes place in the vagina and in the mean time ootheca formation starts during April-June. Clitellum becomes conspicous during ootheca formation. They secrete snow white girdle around clitellum i.e., segment 9 to 11 and clitellar gland produce albumen in the girdle. Now many fertilized eggs are laid into girdle through female genital pore. - Leech now start withdrawing its preclitellar region from the anterior side of girdle. The prostomium glands secrete an anterior plug to close the anterior pole of the girdle. By further rhythmic contr...

- Nereis is a unisexual polychaet. Their ( and ovaries) appear only during breeding season (Summers) so there are no regular ducts to carry gametes to the exterior. Their gametes are discharged through temporary aperture formed in the dorsal body wall in gonad bearing segment or through nephridiopore. - Nereis undergo remarkable changes in external form and behaviour during breeding season called swarming e.g., eyes enlarge, parapodia of posterior half get modified into swimming paddles, older setae are replaced by large, numerous setae. The posterior half of the body is called epitoke and anterior half with normal parapodia is atoke. This shape of Nereis is called Heteronereis. Life History of Nereis Show Following Stages : - The newly discharged eggs are rich in oil droplets and yolk spherules. They are covered by zona radiata. - Fertilization takes place in sea water and results in disappearance of zona radiata. - - Cleavage in Nereis is spiral. The first two cleavages are vertical ...

 - In Obelia medusae bear gonads, serve to bring about sexual reproduction and dispersal of species. Both sperm and egg are shed into the sea water by breaking of the epidermis of the gonads which results in the death of medusae. - The life history of Obelia show following stages : By chance both sperm and ova comes in contact in the sea water and results in external fertilization. After fertilization, zygote undergoes holoblastic cleavage and forms blastomeres. It converts zygote into hollow spherical blastula enclosing cavity, blastocoel. - Some of the blastomeres cut off from inner surface and immigrate to opposite pole in blastocoel. This process is called delamination. Immigrated cells ultimately fill up the blastocoel. Outer layer of the cells is termed as ectoderm and inner mass of cells form endoderm. This embryonic stage is called stereogastrula i.e., solid gastrula. - Gastrula elongates and its ectodermal cells acquire flagella. This stage is called planula. It has broad ...

Any part of earth's surface that supports vegetation also bears a covering of soil. Soil is thus usually defined as "any part of earth's crust in which plants root". Muddy bottoms of ponds, porous rock surfaces, ravines or glacial deposits, bottoms of lakes, peats etc., all are thus soils. But this is a limited definition of soil, as we know that soil is actually formed as a result of long-term process of complex interactions leading to the production of a mineral matrix in close association with interstitial organic matter-living as well as dead. In soil formation, modification of parent mineral matter takes fairly a long time. Such a modification is actually the result of interactions between climatic, topographic and biological effects. Soil is thus not merely a group of mineral particles. It has also a biological system of living organisms as well as some other components. It is thus preferred to call it a soil complex, which has the following five categories of c...

 Animals are metazoans that differ from plants in a number of characteristics like absence of plastids, cell wall and central vacuole. There are about 1.2 million animal species which show great diversity in shape, size, colour, habits, habitat, structures etc.  CHARACTERISTICS OF ANIMALIA   - They are multicellular consumers of the ecosystem. - They are made up of eukaryotic cells and have no cell wall, plastids, central vacuole and photosynthetic pigments. - Lower animals exhibit cellular or tissue level of organization and higher animals have organ system level of organization. - Mostly free living and some (sponges and several coelenterates) are fixed.  - They take food by ingestion i.e., holozoic in nutrition and digestion takes place in an internal cavity. Some forms have absorptive nutrition and their digestive cavity is absent. The undigested food is removed by egestion. -They have muscle cells (contraction and relaxation  of body parts) and nerve cells ...

Evolution can be regarded as the effect of natural selection on the continous appearance of mutant forms. It is a modified version of natural selection and mutation. Postulates 1. Genetic Variability 2. Natural selection  3. Reproductive Isolation 4. Speciation 1. Genetic Variability Variations are essential for evolution. There are number of sources of genetic variability : - Chromosomal aberrations as inversion, translocation. - Change in chromosomal number as aneuploidy, deletion,duplication,polyploidy. - Gene mutation as substitution, addition, point, frameshift, gross mutations. - Mutagens-HCHO, H₂O2, Mustard,5- bromouracil, colchicine, rays, viruses etc.  - Hybridisation - It is the interbreeding of two genetically different individuals. - - - - Genetic Drift- It is the elimination of genes of some original characteristics of a species due to migration. It operates only in smaller population ie, error in gene pool sample (sampling error). In a new habitat, the colonisers...