(a) Give a comparative account of aortic arches in vertebrates. (20 marks) (b) Explain different ways of locomotion in Protozoa…

(a) Comparative account of the aortic arches in vertebrates (20 marks)

The embryonic vertebrate has six pairs of aortic arches (I–VI), which connect the ventral aorta to the paired dorsal aortae through the pharyngeal (visceral) arches. The ancestral pattern, in which each arch is broken by gill capillaries into afferent and efferent parts, is modified in each class.

• Cyclostomes: arches supply the gill pouches, in more numerous pairs, and each arch is divided into afferent and efferent vessels.
• Fishes: arches I and II are lost or reduced, while arches III to VI (four pairs in bony fishes; five pairs in sharks such as Scoliodon) persist as gill arches. Each breaks up into an afferent branchial artery from the ventral aorta and an efferent branchial artery to the dorsal aorta. Blood flows in a single circuit.
• Amphibians: in the tadpole arches III–VI serve the gills. In the adult arch III becomes the carotid arch (common carotid, external and internal carotid), arch IV the systemic arch to the dorsal aorta, arch V is lost, and arch VI becomes the pulmocutaneous arch to the lungs and skin.
• Reptiles: arch III forms the carotid arches, arch IV persists as paired systemic arches (right and left), arch V is lost and arch VI forms the pulmonary arches. In crocodiles the left systemic arch arises from the right ventricle.
• Birds: arch III forms the carotid, arch IV persists on the right side only as the right systemic arch, the left one being lost, and arch VI forms the pulmonary arches.
• Mammals: arch III forms the carotid, arch IV persists on the left side only as the left systemic (aortic) arch, the right forming the root of the right subclavian, and arch VI forms the pulmonary arch. In the foetus a duct (ductus arteriosus) joins the left pulmonary arch to the aorta, and it closes after birth into the ligamentum arteriosum.

Trend. From fish to mammals the number of arches falls, the gill function disappears, and the arches are remodelled to separate the pulmonary and systemic circulations with a single systemic arch in the higher forms.

(b) Modes of locomotion in Protozoa (15 marks)

• Amoeboid (pseudopodial): the cytoplasm flows in a sol–gel cycle, with actin and myosin, to form pseudopodia. There are lobopodia in Amoeba, filopodia in Euglypha, reticulopodia in foraminifera such as Elphidium, and axopodia in Actinophrys.
• Flagellar: one or more flagella, each with a 9 + 2 axoneme, beat in waves and pull or push the cell forward, as in Euglena, Trypanosoma and Noctiluca.
• Ciliary: numerous short cilia beat in an effective stroke and a recovery stroke, and the coordinated metachronal waves move the cell, as in Paramecium.
• Metabolic (euglenoid): the flexible pellicle allows the body to contract and elongate in waves, as in Euglena.
• Gliding: the cell glides over a surface without visible organelles, by a secreted film of mucilage and the movement of the cortex, as in gregarines, and in the sporozoites of Plasmodium.
• Contractile: contractile fibres called myonemes shorten the body or stalk, as in the stalk of Vorticella and in Stentor.
• Passive floating: some planktonic forms, such as radiolarians, float, aided by oil droplets and vacuoles.

(c) General features and life history of Nereis (15 marks)

General features. Nereis (the clamworm) is a marine annelid that lives in burrows in sand and mud in coastal waters and is a predator or omnivore.

• The body is elongated, segmented and up to 200 segments long, with a head made of the prostomium, with two pairs of eyes, two tentacles and two palps, and a peristomium with four pairs of tentacular cirri.
• Each trunk segment bears a pair of biramous parapodia, with acicula, setae and dorsal and ventral cirri, used for swimming, crawling and respiration.
• The gut has a muscular pharynx (proboscis) with a pair of jaws. The coelom is schizocoelic, and the blood-vascular system is closed with haemoglobin in the plasma. Nephridia excrete, and the nervous system consists of a brain and a ventral ganglionated cord.
• Sexes are separate, with no permanent gonads.

Life history.

• At maturity the worm becomes the heteronereis (epitoky): the eyes enlarge and the parapodia are flattened into paddles, so the worm can swim to the surface.
• It swarms, often in relation to the moon, and eggs and sperm are shed into the sea, where fertilisation is external. The adults die after spawning.
• Cleavage is spiral and holoblastic, giving a blastula and gastrula, and a free-swimming trochophore larva, then a metatrochophore and a nectochaete, which settles as the young worm.