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Describe the General Plan of the. Circulatory or Vascular 

 System. 



There are two main longitudinal vessels, one (the dorsal 

 aorta) above the alimentary canal, and one (ventral vessel) 

 below it, connected by a series of half-loop-like vessels 

 (aortic arches) round each side of the pharynx. The blood 

 system of an Annelid worm is fundamentally similar. 

 That portion of the ventral vessel " within the limits of 

 the pericardiac coelome " is the cardiac tube, and it becomes 

 the heart within the pericardium ; the portion in front of 

 the heart is the ventral aorta, and the portion behind is the 

 subintestinal vein. 



State what you kyow about the Development of the Heart and 

 Blood-vessels, and indicate the General Structure of 

 the Heart. 



The muscular walls of the heart and principal vessels 

 (veins and arteries) are developed from splanchnic meso- 

 derm, and are lined internally with endothelium (i.e., 

 mesenchyme cells which have become flattened and united 

 to form a delicate membrane). The walls of the capillaries 

 consist of endothelium only. 



The cardiac tube (see previous answer) becomes kinked, 

 and develops four successive chambers, varying in diameter 

 and thickness of wall, namely, the posterior sinus venosus, 

 the atrium or auricle (receiving chamber), overlying the 

 thick-walled ventricle (pumping chamber), and the thick- 

 Availed anterior conus arteriosus, which may be absent. 

 Such is the primitive heart in Fishes ; and (except in 

 Dipnoi) it contains impure blood only. In Amphibians 

 the atrium is divided by a septum into two auricles, and 

 in Crocodiles, Birds, and Mammals two ventricles are like- 

 wise formed ; therefore the heart is practically double in 

 the higher Craniata, and it contains (in its right half) impure 

 and (in its left or systemic half) pure blood. The chambers 

 of the heart are separated by valves, which permit flow of 

 blood only in the direction from auricle to ventricle. 



Mention the General Features of the Blood Circulation. 



In Fishes the conus (or a non-contractile bulbus) leads 

 into a ventral aorta, which gives off on each side a series 

 of ascending branches (afferent branchials) conveying the 

 impure blood to the gills. The blood returns direct to the 



P 



