VI 



NEMEETINEA 



121 



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

 radial muscles which serve to contract and elevate the lappets (Fig. 96). 



Fig. 95. — Two stages in the development of the Pilidium larva of Cerebratulus lacieus. 

 Showing tlie development of mesenchyme into mnscles. A, Earlier stage. B, Later 

 stage. (After C. Wilson.) 



I, lappets of prototrocli ; oc, oesophagus ; -s?, stomach. Other letters as before. 



muscper 



Fig. 96. — Two views of advanced Pilidium larva of Cerebratulus lacteus to .show the 



development of the muscles. (After Wilson, ) 



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



muscper, 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 apjjear all over its wall. 



