Plate 21. 



Microphotographs of thick unstained preparations, showing skeletal arrangement. 

 Fig. 1. Polymastia maeandria. Dermal surface ; x 50. 

 " 2. Petrosia variabilis crassa. " " x 20. 



" 3. " " " Section vertical to surface, which is represented by 



upper margin of figure ; x 15. 

 " 4. Petrosia si))iilis densissitna. Section vertical to surface, which is represented by 



upper margin of figure; x 15. 

 " 5. " " " Dermal surface ; x 20. 



" 6. Gellius jx.rforatus. Dermal surface ; x 25. 



