(a) Explain the general features, life cycle and pathogenicity of Taenia. (20 marks) (b) Give an illustrated account of origin of…

**(a) General features, life cycle and pathogenicity of *Taenia***

General features (Taenia solium, the pork tapeworm): Habitat and form: an endoparasite in the human small intestine, 2 to 3 m long, flat, ribbon-like, and acoelomate. Scolex: the anterior attachment organ, with four suckers and a rostellum bearing two rows of hooks; the beef tapeworm T. saginata has no hooks. Body: the neck buds off the strobila of several hundred proglottids. Immature, mature and gravid proglottids are arranged from front to back. Adaptations: no gut, as food is absorbed through the tegument, and hermaphrodite proglottids with large reproductive organs. Life cycle (two hosts): Step 1: Gravid proglottids, each full of embryonated eggs, leave with the faeces of man, the definitive host. Step 2: Eggs are swallowed by the pig, the intermediate host. In its intestine the six-hooked oncosphere hatches, bores through the gut wall and is carried by blood to the muscles. Step 3: In muscle it grows into a bladder worm, the cysticercus (Cysticercus cellulosae), with an inverted scolex. Infected meat is called measly pork. Step 4: Man eats undercooked measly pork. In the intestine the scolex turns out, attaches itself, and grows into the adult in two to three months. Pathogenicity: Intestinal taeniasis causes abdominal pain, indigestion, loss of weight and mild eosinophilia. Cysticercosis follows when man swallows the eggs of T. solium, through contaminated food or by autoinfection. Cysticerci lodge in muscles, eyes and brain. Neurocysticercosis causes headache, seizures and raised intracranial pressure, and is a common cause of epilepsy in India. Prevention: cooking pork properly, sanitary disposal of faeces, meat inspection and treatment of carriers.

(b) Origin of mammals

Mammals evolved from the synapsid line of reptiles, which split from other reptiles in the Carboniferous. Evolutionary sequence: Pelycosaurs (Dimetrodon) of the Carboniferous and Permian had one lower temporal opening and sprawling limbs. Therapsids of the Middle Permian had differentiated teeth, a more erect posture and a larger dentary. Cynodonts (Cynognathus, Thrinaxodon) of the late Permian and Triassic were the closest relatives of mammals. Mammaliaformes such as Morganucodon of the late Triassic were small, shrew-like and nocturnal, and gave rise to the true mammals. Changes in the line: Jaw: the dentary enlarged and finally formed a joint with the squamosal, while the articular and quadrate bones moved into the middle ear as malleus and incus. Teeth: differentiation into incisors, canines, premolars and molars. Palate: a secondary palate separating food and air passages, allowing breathing while chewing. Other changes: double occipital condyle, diaphragm, a larger brain, hair and a warm-blooded condition. Evidence: fossil series of cynodonts showing the gradual transfer of jaw bones, and the persistence of the two ear ossicles in the embryonic mammalian jaw. Diagram: a flow diagram of the lineage from pelycosaurs through therapsids and cynodonts to mammals, with the jaw change.

**(c) Perivisceral coelom and endoskeleton of *Asterias***

Perivisceral coelom: It is a large enterocoelic cavity lined by ciliated peritoneum and filled with coelomic fluid containing amoebocytes (coelomocytes). It surrounds the stomach, pyloric caeca and gonads and extends into each arm, where it is divided by mesenteries. The fluid circulates by ciliary action and takes part in transport of food and oxygen, and in excretion, since amoebocytes carry wastes to the papulae. The papulae (dermal branchiae) are thin outgrowths of the body wall, containing coelom, which serve for respiration. Endoskeleton: It is mesodermal and lies in the dermis, formed of calcareous ossicles (calcite) joined by connective tissue. Ossicles are arranged in a network on the aboral surface and bear spines. Marginal plates edge the arms. In the ambulacral groove the ambulacral ossicles form a V-shaped ridge on each side. They allow the arm to bend and give attachment to the muscles of the tube feet. Pedicellariae are tiny pincer-like organs on the skeleton, which keep the surface clean of debris and larvae.