706 



ZOOLOGY 



becomes differentiated into a secreting portion or kidney whicl 

 is very spongy in texture, and opens into the pericardiun 

 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 excretion are the pericardial glands, or Kebcrs 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, h) 

 and some other genera ; the ordinary arrangement seems to have 



Fio. 590. Sagittal section of part of enteric canal of Dcnax. I, lower lip ; II, intestine ; III, 

 pyloric csecum ; IV, crystalline style ; V, cuticle ; VI, stomach ; VII, gullet ; 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 constriction. 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 case : the blood-system is everywhere closed. The blood 

 is red in some forms (e. g., Area) owing to haemoglobin in the 

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

 presence of hsemocyanin. 



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 copnective with the pedal, 



