706 



ZOOLOGY 



becomes differentiated into a secreting portion or kidney — which 

 is \ery spongy in texture, and opens into the pericardium, 

 and a non-secretory portion or bladder, which opens externally. 

 Frequently there is a communication between the right and left 

 nephridia, and in some genera, such as the Oyster, the organs 

 become extensively branched. Also taking a share in the process 

 of exci'etion are the 'pericardial glands, or Keber's organs, glandular 

 developments of the wall of the pericardium. 



Circulatory Organs. — The heart is usually perforated by 

 the rectum, but lies altogether, above it in Nucula (Fig. 586, A) 

 and some other genera ; the ordinary arrangement seems to have 



Fig. fjOO.— Sagittal section of part of enteric- c-anal of Donax. I, lower Up ; 11, intestine ; 111, 

 pyloric cfecmn ; IV, crystalline style; V, cuticle; A'l, stomach; VII, giiUet ; VIII, upper 

 lip ; IX, mouth. (From Pelseneer.) 



been brought about by the heart becoming folded over the intes- 

 tine and united below it. In the Oyster and some other forms the 

 heart is below the rectum. In Area the ventricle is divided into 

 two by a constrictifTii. Pores are often found on the surface 

 of the foot, and it has been asserted that through them the 

 external water mixes with the blood ; this, however, is certainly 

 not the ease : the blood-system is everywhere closed. The. blood 

 is red in some forms («. g., Area) owing to hsemoglobin in the 

 corpuscles ; in some cases it is of a bluish tint owing to the 

 presence of ha;mocyanin. 



The nervous system is found in its most primitive condition 

 in Nucula (Fig. 591). Instead of a single cerebro-pleural ganglion 

 there are, on each side, distinct cerebral (XVI) and pleural (I) 

 ganglia, each united by a connective wjth the pedal, 



