COMMON CRAYFISH. 179 



ante, p. 170) as far forwards as the oesophagus. It is by means of this 

 muscle, by which the abdomen is suddenly flexed, that the animal executes 

 its rapid backward darts through the water. The ambulatory thoracic limbs 

 are employed for the slower movements of crawling ; and the muscles that 

 move these limbs on the right side may be seen passing through the 

 intervals of the endophragmal skeleton from their origins upon the epimera 

 displayed in Preparation 36. 



34. COMMON CRAYFISH (Astacus fluviatilis\ 



Dissected to show the heart and the origins of the chief vessels in situ. 



THE cardiac region of the omostegite and the adjoining region of the 

 cephalostegite have been removed, as well as the tergal regions of the two 

 first abdominal somites. The walls of an arterial pericardial sinus which 

 surrounds the heart have also been removed, and that organ with the origins 

 of the principal vessels has been consequently exposed. The heart is 

 hexagonal in outline and compressed. Five arteries spring from its an- 

 terior border. One in the middle line supplies the eyes and first antennae. 

 Two others, one on each side this median artery, pass obliquely forward 

 to the second antennae. And finally, two hepatic arteries, not seen here, 

 spring one from each outer and inferior angle and pass downwards between 

 the anterior testicular lobes and the intestine to the liver. One of them is 

 shown in Prep. 33. A single dilated trunk takes origin from the posterior 

 border of the heart. It divides at once into two branches. One of these, 

 the superior abdominal artery, lies immediately above the intestine in the 

 median dorsal line. A slip of blue paper has been placed under it. The 

 other, the sternal artery, seen in Prep. 33, passes between the commissures 

 uniting the third and fourth thoracic ganglia. It then divides, and one 

 division passes forwards, the other backwards beneath the nerve-cord 

 as the inferior abdominal artery. The muscular walls of the heart are 

 pierced by six main valved inlets, which permit blood to enter but not 

 to pass back. Two of these inlets are on the dorsal surface and are visible 

 in this preparation about the middle region. Two other inlets are ventral, 

 and the remaining two are placed, one on the right, the other on the left 

 side. In addition to these main inlets, Dezso has described others of small 

 size, four pairs on the dorsal and two pairs on the ventral surface. Six 

 elastic alae cordis connect the heart to the non-muscular pericardium. 

 Their main function is probably to antagonise the contractions of the heart 

 itself. They may also serve to suspend it in the sinus, but the arteries are 

 probably, in this as in other animals, the chief means by which it is kept in 

 position. 



The polygonal aspect of the Decapod heart and the presence of 



N a 



