Cleidoic eggs have a protective shell, and are laid out of water. The shell is porous to air, and may be flexible or calcareous (hard). The eggs contain all the food the embryo needs to develop into a hatchling. The significance of the cleidoic egg is that it enables reproduction out of, and often away from, water. It is a characteristic of birds and reptiles. Hence, option D is correct.

The notochord is the defining structure forming in all chordate embryos. In humans beings, the notochord is formed in the third week and eventually replaced by the vertebral column. 

Schizocoelom develops as a split in the mesoderm, e.g.-Arthropods, Annelids, and Molluscs. Enterocoelom is derived from the wall of embryonic gut, e.g.-Echinoderms, and Chordates. Haemocoel is the blood-filled cavity of arthropods. Pseudocoelom is also known as a false cavity.

Ascariasis is a disease caused by the parasitic roundworm Ascaris lumbricoides. These are long, slender, tapered at both the ends. The body is covered with a thick cuticle secreted by a cellular or syncytial epidermis. The life cycle of Ascaris involves only one host. The man is a definitive host for worm and is present in the gastrointestinal tract and fecal matter. Hence, option A is the correct answer. 

Important class of waterborne diseases are caused by metazoan parasites. Typical examples include certain Nematode, that is to say "roundworms". As an example of waterborne Nematode infections, one important waterborne nematodal disease is Dracunculiasis. It is acquired by swallowing water. Therefore, the correct answer is option A.

Platyhelminthes or flatworms are dorsoventrally flattened and bilaterally symmetrical worms. They are endoparasites and have parasitic adaptations like hooks and suckers.

• The English scientist Edward Jenner (1749 - 1823), regarded as the "Father of Immunology", first introduced the "Smallpox vaccine" in 1796. In fact, the Smallpox vaccine was the first successful vaccine to be invented. Smallpox is a serious contagious disease, caused by the Variola virus. The infected people can have a high fever, body ache, vomiting, and distinctive skin rashes. The smallpox vaccine comprises of live "Vaccinia virus", which quite similar to the Smallpox virus. Jenner's invention eventually led to worldwide immunization by the Smallpox vaccine and eradication of the infectious disease. 
• Heredity refers to the passing of genetic information from parents to their offspring. Genes are the basic structural and functional units of heredity. In eukaryotic organisms, the Genes remain packaged into large nuclear components, called the Chromosomes. Therefore, each Chromosome comprises many genes. That is why the chromosome is regarded as the "carrier of heredity".   
  
• The Coelenterate animal Hydra can reproduce either asexually or sexually. Sexual reproduction in Hydra takes place through the formation of gametes. Asexual reproduction in Hydra is usually seen in the month of summer, especially when the animal is healthy and well-fed. Hydra reproduces asexually by means of Budding. Repeated multiplication of its body-wall cells, results in a bulging appearance near the middle, or the basal part of its body. This bulge grows as a Bud. The wall of the Bud is like an extension of the epidermis and gastrodermis the parent Hydra, while the lumen of the Bud is in continuation with the parent's gastrovascular cavity. A fully grown up Bud finally detaches from the parent's body and gradually develops into an adult Hydra.

Peripatus is sometimes considered a living fossil because it and other onychophorans are transitional between Arthropoda and Annelida and retain a large. So, the correct option is "A" (Peripatus).