l8o ELEMENTARY BIOLOGY. [CHAP. 



of a digestive ferment ; as these physiological activities are 

 frequently relegated respectively to separate liver and 

 pancreas, the single gland which here performs the two 

 functions has been appropriately termed the hepato-pancreas. 



The heart is a short, thick, somewhat hexagonal, symme- 

 trical organ lodged in the pericardiac sinus, to the walls of 

 which it is attached by fibrous bands. In its anterior half 

 three 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, mainly 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 ab- 

 dominal artery, runs along the dorsal face of the intes- 

 tine, giving off transverse branches as it goes, in each 

 somite ; the other, the sternal artery, immediately on leaving 

 the pericardial -sinus distributes branches to the genital 

 gland; it then passes ventrally, to the interspace between 

 the penultimate and antepenultimate thoracic ganglia, 

 passes between their commissures and divides into two 



