VI 



NEMEKTINEA 



121 



form of a fenestratecl sheet. In the lappets the peritoneal muscles form 

 radial muscles which serve to contract and elevate the lappets (Fig. 9G). 



B 



mes 



OC ' 



. ^m&mmtimmw?'^ "v ; -mtmam 



'Ifjprv 



FIG. 95. Two stages in the development of the Piliclium larva of Cerebratuhis lacteus. 

 Showing the development of mesenchyme into muscles. A, Earlier stage. B, Later 

 stage. (After C. Wilson.) 



/, lappets of prototroch ; oe, oesophagus ; nt, stomach. Other letters as before. 



fniiscper 



FIG. 96. Two views of advanced Pilidium larva of Cerebratulus lacteus to show Ihr 



development of the muscles. (After Wilson. ) 



A, viewed as a transparent object. B, surface view, am, anterior amniotic invagination ; 

 musc.per, so-called peritoneal muscles ; ret, retractor of the apical plate. 



On the posterior wall of the oesophagus a groove for conducting 

 food appears, and numerous gland cells appear all over its wall. 



