a. A. DREW ON YOLDIA LIMATULA, SAY. 25 



ends, which penetrate the mantle, and are attached to the shell. The 

 posterior adductor muscle, Figs. 33 and 55, pa, does not appear until 

 some hours later. It is always surrounded by other tissue. The 

 digestive glands, Figs. 33, 62 and 63, are formed as evaginations from 

 the anterior end of the mid-gut, which now extends posteriorly 

 nearly to the blastopore. The visceral ganglia appear as groups of 

 nuclei, whose position suggests ectodermal origin. The cerebral 

 pouches have moved some distance dorsally and posteriorly into the 

 interior of the embryo, and are situated at the end of a single pouch, 

 Figs. 33, r, which opens to the exterior between the test-cells at the 

 point where the pouches were originally formed. The paired pouches 

 thus appear as diverticula of a single elongated pouch. As the 

 pouches are carried into the interior, the cells forming their walls 

 push in and fill up their cavities, and a commissure is formed con- 

 necting the two masses of cells. The relation of the unpaired pouch 

 to the cerebral ganglia has been shown by three succeeding transverse 

 sections of the same embryo. Fig. 58 shows the external opening of the 

 unpaired pouch, Fig. 59 shows the anterior portion of the cerebral 

 ganglia, eg, with the laterally compressed unpaired pouch, r, lying 

 beneath them, and Fig. 60 shows the commissure connecting the 

 ganglia. 



Having described the formation of the various organs attention will 

 be given to the embryo's appearance. 



Further than the lateral compression and the corresponding dorso- 

 ventral widening, there has been little change in the embryo's external 

 appearance, but the internal appearance is greatly altered. 



Inside the test a shell has been formed, the two valves of which 

 gape ventrally. The anterior adductor muscle, Fig. 33, aa, is quite large 

 and, being isolated from the general mass of the body, is very con- 

 spicuous. The posterior adductor muscle, pa, as yet consists of very 

 few fibres and is inclosed in other tissue. It is not easily seen in whole 

 mounts. The apical plate, ap, is connected with the 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 to 

 connect with the anterior part of the body. The mid-gut, mg, and the 

 stomod^eum, std, are widely separated by the foot, f. The stomodaeum 

 opens to the exterior through the blastopore, bl, and the posterior end of 

 the mid-gut almost reaches, and finally breaks through into, it. 



The stomod^um is often attached to the anterior, and to part of 



