332 OILMAN A. DREW. 



Tlie heai't (fig. 41, h.) is apparently formed from meso- 

 dermal tissue that collects to form a strand, that rims across 

 the pericardium from oue side to the other. I have found no 

 indication of its being" formed as paired pouches, as described 

 by Ziegler (20) for Cyclas cornea, nor have I found any 

 evidence that it originates as two masses that grow toward 

 each other. Its first appearance seems to be in the form of a 

 mesodermal strand of tissue that soon hollows out and 

 encloses the intestine. The fact that the heart forms around 

 the intestine, and not dorsal to it, is of interest, and will be 

 discussed under the head of the Circulatory System. 



The growth of the kidneys, which are now present as 

 small tubes, seems later to force the sides of the heart up 

 around the intestine (fig. 68), so that the ventral portion ot" 

 the ventricle becomes drawn out into a trough in which the 

 intestine lies. As the kidneys grow the trough becomes 

 deeper. By gradually closing in dorsal to the intestine at 

 the anterior and posterior ends, the trough is shortened, and 

 the intestine finally becomes free from the heart and lies 

 ventral to it (fig. 69). This is accomplished by a very slow 

 process, and is not completed until after the animal has 

 become sexually mature. 



I am inclined toward the opinion that the kidneys are 

 formed by the differentiation of mesodermal tissue. When 

 they first appear each is a very narrow tube, and extends 

 from its external opening in the mantle chamber to the mid- 

 line of the body. I have not succeeded in demonstrating 

 the inner pericardial openings of the kidneys in this or in 

 later stages. The cells soon become large and vacuolated, 

 and the kidneys grow rapidly and crowd anteriorly ventral 

 to the pericardium, where they become coiled and sac- 

 culated. 



With the formation of the fourth lobe of the gill (fig. 45) 

 processes make their appearance on the bases of the lobes, 

 between them and the mantle lobe to which the gill is 

 attached. These processes grow to form what have been 

 called the outer gill plates, but in this species their position 



