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retina of such a type as is present here, the several eyes 

 will have but a limited area from which rays can be 

 focussed on the receptive surface. I have never observed 

 the eyes of Pecten beino- moved in various directions, they 

 can only form an imap^e of some object directly in front of 

 them. This will account for the need of such a number 

 of eyes, if they are to be of real use as visual organs. 



EXCRETOEY SYSTEM. 



The most important renal excretory organs are the 

 paired glands (fig. 1, R. o.) lying at the sides of the 

 visceral mass, and sometimes known as nephridia. They 

 were formerly termed the organs of Bojanus, after their 

 discoverer, who first described them in 1819. 



Morphologically, they are like nephridia only in as 

 far as they open on the one hand to the exterior and on 

 the other hand into the pericardium — the remains of the 

 coelom. But, since they are out-growths from the 

 coelom, they are true coelomoducts. 



In Pecten, they are elongated pouches of a light to 

 dark brown colour. They are attached to the anterior 

 surface of the adductor muscle, on each side of the 

 visceral mass, and lie between the latter and the ctenidia, 

 extending over the muscle for about 90° from the region 

 of the digestive gland to near the last point of attachment 

 of the visceral mass. 



They are slightly asymmetrical, the left being the 

 larger of the two, and this difference is correlated with the 

 position of the visceral mass. The organ lies directly on 

 the adductor, the glandular walls being separated from it 

 by connective tissue. The outer wall of the organ of 

 Bojanus is formed by a direct continuation of the 

 epithelium of the visceral mass over it, on its way to form 



H 



