511 



enlarge to form the stomachs, and extend posteriorly as intestines. 

 The cerebral pouches of Yoldia move some distance dorsally and 

 posteriorly, and are situated at the end of a single pouch, which opens 

 at the surface, between the test-cells, where the pouches were origin- 

 ally formed. The paired pouches are thus brought to lie in the interior, 

 as diverticula of a single, elongated pouch. As they are carried into 

 the interior, the cells that form their walls divide and fill up their 

 cavities, and a commissure is formed connecting the two masses of cells. 

 The embryos of Yoldia, when from 80 to 100 hours old, are later- 

 ally compressed, but are otherwise not much changed in external 

 appearance. 



Fig. 16. Reconstruction of an embryo of Yoldia limatula, at a stage just before 

 the test is cast off. The embryo is seen from the left side. Specimens are 0,2 mm long, 

 without the apical cilia. aa anterior adductor muscle, ac apical cilia, bl blastopore. 

 eg cerebral ganglion. / foot, int intestine. II left lobe of the digestive gland, ot oto- 

 cyst. pa posterior adductor muscle, pg pedal ganglion, r pouch leading from the sur- 

 face to the cerebral ganglia, s shell, std stomodseum. t test, vg visceral ganglion. 



Inside the test, Fig. 16 t, a shell, s, has been formed, the two 

 valves of which gape ventrally. The anterior adductor muscle, aa, is 

 quite large and, being isolated from the general mass of the body, is 

 very conspicuous. The posterior adductor muscle, pa, as yet consists 

 of very few fibers, and is enclosed in other tissue. It is not easily 

 seen in whole mounts. The apical plate, bearing the tuft of apical 

 cilia, ac, is connected with the antero - ventral end of the group of 

 cells that form the wall of the unpaired pouch, r. This group of cells 

 forms a distinct stalk, which runs dorsally and posteriorly, and con- 

 nects with the anterior part of the body. The intestine, hit, and the 

 stomodseum, std, are widely separated by the foot, f. They both open 



34* 



