510 



generally open to the exterior together, Fig. 15 eg, but they may open 

 between different test-cells. In some cases only one of the pouches 

 opens to the exterior, and possibly in some embryos there is no opening 



at all. The walls of the 

 pouches are rather thick 

 but they are composed 

 of single layers of cells. 

 The cells from which 

 the cerebral ganglia of 

 Nucula are formed, do 

 not have a separate sur- 

 face connection , and 

 nothing resembling the 

 cerebral pouches of Yol- 

 dia have been observed. 

 In all three forms, 

 the pedal, and later the 



Fig. 15. Transverse section of a 45 hour embryo of Yoldia limatula, taken through 

 the cerebral pouches, eg cerebral pouches, mg anterior wall of the mid-gut. t test. 



visceral ganglia are formed as ectodermal thickenings. The otocysts 

 appear, as ectodermal invaginations, about the time that the pedal 

 ganglia form. With Yoldia this is while the embryos are free swim- 

 ming. With both species of Nucula it is at a later period. 



The formation of the mantle and shell is accompanied by a slight 

 lateral compression, and a corresponding dorso - ventral widening of 

 the embryos. The test-cells become much thinner, and the vacuoles, 

 which have been especially large and numerous in the test -cells of 

 Yoldia, nearly or quite disappear. The cilia remain active and the 

 movements of swimming are not altered. Cleared, whole mounts of 

 embryos at this stage, show some of the internal organs, but it is 

 necessary to resort to reconstructions from the study of serial sections, 

 to get the relationships of the different parts. 



The bodies of the embryos now lie between the shell-valves, which, 

 in turn, are enclosed by the test. 



The foot of Yoldia is formed by a rapid increase of cells between 

 the mid-gut and the stomodaeuin. The formation of the foot of Nucula 

 is delayed to a later time. The anterior adductor muscles stretch 

 between the shell - valves, and occupy rather isolated positions. The 

 posterior adductor muscles appear somewhat later. The digestive 

 glands are formed as evaginations from the mid-guts. The mid-guts 



