68 



ESSENTIALS OF ZOOLOGY 



Pericard. gland 

 ^ Gonad opening 

 ^Ureter opening 



Ureter 



Kidney 



Pericardium 



anteriorly. The upper of these openings is that of the coelomi- 

 duct, and the lower is that of the gonad. 



Between the two openings the coelomiduct is disposed in 

 a loop. From the inner opening the duct passes posteriorly 

 to the posterior adductor and is dark in colour, the epithelium 

 being glandular and thrown into folds. This tube removes 

 the excretory material from the blood. At its posterior end 

 it gives off dorsally the thin- walled portion of the tube, which 

 extends forwards over the glandular portion to open as stated 

 by the external aperture. This thin- walled part of the coelo- 

 miduct is thus inter- 

 posed between the 

 glandular part and the 

 floor of the pericardial 

 cavity, and is fre- 

 quently termed the 

 ureter. The ureters 

 meet and fuse an- 

 teriorly just before they 

 diverge to reach the 

 external openings. It 

 is important to note 

 that the pericardial 

 cavity is a closed one 

 and communicates with 



the exterior only by the coelomiduct. The gonad, which is 

 tubular, is also a derivative of the coelom. 



Vascular System. The blood is colourless, consisting of a 

 plasma slightly tinged with haemocyanin, a respiratory 

 pigment associated with copper, and the blood cells are 

 leucocytes. It circulates in vessels and irregular spaces or 

 lacunae interposed between the ectoderm and the organs. 

 The heart is primitively dorsal in position. The ventricle, 

 however, is enclosed in a fold of the pericardial cavity, and it 

 is besides wrapped round the rectum, which thus appears to 

 pierce it. Arterial blood is conveyed to the ventricle from 

 the gills by a pair of delicate auricles. The auricles are 

 broad at their lateral borders where they originate from the 

 lateral wall of the pericardial cavity, and they narrow as 



FIG. 32. Anodonta. Diagram of pericardial 

 cavity and coelomiduct. 



