xii.] THE FRESH-WATER CRAYFISH. 133 



intestine is slender and delicate, smooth internally in the 

 Lobster, papillose in the Crayfish. Near its hinder end its 

 walls become thicker for a short distance, and this, thick- 

 ened portion, with which, in the Lobster, a short dorsal 

 caecum is connected, may be regarded as the large intestine 

 or rectum. 



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

 cal organ lodged in the pericardiac sinus, to the walls o'f 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, 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 transverse branches as it goes, in each somite ; and 

 the other, the sternal artery, which passes ventrally to the 

 interspace between the penultimate and antepenultimate 



