46. 



Having described the for!:aatioii of the various organs 

 atteiition will be given to tlie ei3ibr;;-o*s appcara;ice. 



I^irtlier than tlie lateral co:tipressiori and th.e correspond- 

 ing dors o -ventral widening; tliere lias been little change in the 

 embryo's external appearance, but the internal appearance is 

 greatly altered. 



Inside the test a shell has heeri formed, the two valves 

 of which gape ventral I7. The anterior adductor inuscle, figure 33, 

 aa, is quite large and, being isolated from tiae general niass of 

 the body is very conspicuous. The posterior adductor rmiscle, 

 pa, as yet consists of very few fibers and is enclosed in other 

 tissue. It is not easily seen in Tidiole 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 groiip of 

 cells f orraS a distinct stalk w^iich runs dorsally and posteriorly 

 to connect with the anterior part of the body. The midgut, ng, 

 and the stomodaeun, std, are widely separated by the foot, f. 

 The stomodaeum opens to the e:coerior through the blastopore, bl, 

 and the posterior end of the midgut almost reaches it but does 

 not open. # 



# Observations since my publication in the Johns Hopkins Uni- 

 versity Circular (-) have not revealed the actiial opening, 

 up to the tine the test is cast away. 



