(a) Define double circulation. Give a comparative account of heart in reptiles, birds and mammals. (20 marks) (b) Describe…

Double Circulation and Comparative Heart Anatomy

Double circulation refers to a vascular mechanism wherein blood passes through the heart twice during a single complete circuit of the body, divided into two distinct pathways: the pulmonary circuit, which transports deoxygenated blood to the respiratory organs for oxygenation, and the systemic circuit, which delivers oxygenated blood to the body tissues. This arrangement ensures complete physical separation of oxygenated and deoxygenated blood, maintaining high systemic pressure and metabolic efficiency in homeotherms.

The vertebrate heart exhibits progressive evolutionary advancement from reptiles to birds and mammals, primarily marked by the gradual completion of the interventricular septum. In typical reptiles, the heart is three-chambered with two completely divided atria (sinus venosus reduced) and a single ventricle partially subdivided by an incomplete muscular interventricular septum into cavum venosum, cavum pulmonale, and cavum arteriosum. While some mixing occurs, separation is functionally maintained during systole. Crocodilians represent an exception, possessing a completely four-chambered heart with two atria and two ventricles, where cross-connection persists via the Foramen of Panizza between the left and right systemic arches.

In birds and mammals, the heart achieves complete structural four-chambered organization with two thin-walled atria and two thick-walled ventricles, entirely abolishing the mixing of blood. Sinus venosus and conus arteriosus are fully incorporated into the right atrium and base of major trunks, respectively. Avian hearts retain only the right systemic (aortic) arch arising from the left ventricle, whereas mammalian hearts retain solely the left systemic arch. In both classes, the evolution of a complete interventricular septum supports endothermy by sustaining high metabolic rates and targeted systemic hydrostatic pressures.

Habitat, Habits and Life History of Branchiostoma

Branchiostoma (Amphioxus or lancelet) is a primitive cephalochordate inhabiting shallow, warm, temperate, and tropical coastal waters with clean, coarse, sandy substrates, prominently recorded along Indian coasts including the Gulf of Mannar and Palk Bay.

It exhibits a sedentary, burrowing habit, remaining buried vertically in sand with only its anterior end bearing oral cirri protruding into water. It is a ciliary, microphagous filter-feeder; the beating of cilia on the wheel organ and gill bars creates a continuous water current entering the mouth, where the endostyle secretes mucus to trap food particles, which are then conveyed to the intestine. Locomotion occurs through fish-like lateral undulations driven by myotomes during nocturnal excursions.

The life history of Branchiostoma is simple and marine. Sexes are separate (dioecious) without sexual dimorphism, featuring about twenty-six pairs of gonads lacking gonoducts. Gametes are shed into the atrium and released via the atriopore for external fertilization. Cleavage is complete, equal, and holoblastic, producing a hollow blastula followed by invagination into a gastrula. Development proceeds through a free-swimming, asymmetric, ciliated larva that feeds for several months. Metamorphosis involves extensive tissue remodeling (metaplasia) rather than severe retrogressive degeneration, culminating in the burrowing adult that clearly retains all diagnostic chordate hallmarks: a persistent notochord extending from rostrum to tail, a dorsal hollow nerve cord, and numerous pharyngeal gill slits throughout life.

Life Cycle of Leishmania donovani

Leishmania donovani, the causative protozoan of Visceral Leishmaniasis (Kala-azar), exhibits a digenetic life cycle alternating between a vertebrate host (humans, canines) and an invertebrate vector, the female sandfly (Phlebotomus argentipes).

The parasite exists in two primary morphological stages: the intracellular, oval, non-flagellated amastigote form (Leishman-Donovan body) measuring two to four micrometres within human macrophages, and the extracellular, elongated, flagellated promastigote form measuring fifteen to twenty micrometres in the insect gut.

In humans, the cycle begins when an infected sandfly introduces metacyclic promastigotes during a blood meal. Host macrophages of the reticuloendothelial system engulf the promastigotes via phagocytosis. Inside the phagolysosome, promastigotes transform into amastigotes, which resist enzymatic lysis, multiply by binary fission, and eventually burst the host cell to invade fresh macrophages in the spleen, liver, and bone marrow.

When a sandfly feeds on an infected individual, it ingests amastigotes. In the insect midgut, amastigotes transform into procyclic promastigotes, multiply by longitudinal binary fission, and migrate to the pharynx and proboscis as infectious metacyclic promastigotes within six to nine days, completing the transmission cycle.

This pathogenic cycle underpins endemic visceral leishmaniasis across the Gangetic plains of India, notably Bihar, Jharkhand, and West Bengal, where targeted sandfly vector control and early antiparasitic therapy remain central to the National Vector Borne Disease Control Programme elimination mandate.