128 ELEMENTARY BIOLOGY. [XII. 



pairs of apertures are visible, two being placed upon the 

 upper face, two at the sides, and two on the under face. The 

 lateral apertures are the most posterior, the dorsal, the most 

 anterior in position. Each aperture begins in a funnel- 

 shaped depression of the outer face of the organ, which leads 

 obliquely inwards and terminates by a valvular slit in the 

 cavity of the heart. This cavity is very much reduced by 

 the encroachment of the muscular bands which constitute 

 the walls of the heart, so that a transverse or longitudinal 

 section shews only a small median cavity surrounded by a 

 thick and spongy wall. 



During life, the heart beats vigorously, the whole of its 

 parietes contracting together. From the dorsal part of its 

 anterior extremity three arteries are given off, one median 

 and two lateral, to the cephalon and its contents, and from the 

 ventral aspect of this end of the heart an hepatic artery is given 

 off, on each side, to the liver. At its posterior end, the heart 

 ends in a median dilatation from which two great arterial 

 trunks are given off; one the superior abdominal artery, which 

 runs along the dorsal face of the intestine, giving off trans- 

 verse branches as it goes, in each somite ; and the other, the 

 sternal artery, which passes ventrally to the interspa3e be- 

 tween the penultimate and antepenultimate thoracic gan- 

 glia, passes between their commissures and divides into two 

 branches, which run, backwards and forwards, between the 

 ganglionic chain and the exoskeleton. 



These arteries divide and subdivide and end in what, in 

 some parts of the body at any rate, e. g. the liver, is a true 

 capillary system. The veins are irregular channels, or sinuses, 

 which lie between the several muscles and viscera. One of 

 the largest of these is situated in the median ventral line, and 

 can be readily laid open by piercing the 'soft integument 

 which lies between any two of the abdominal sterna. The 

 blood flows out of the aperture with great rapidity, and the 



