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Plate 21. 



Microphotographs of thick unstained preparations, showing skeletal arrangement. 



Fig. 1. 



« 



a 



Polymastia maeandria* Dermal surface; x 50. 



2- Petrosia variabilis crassa. 



3 



u 



x 20. 



Section vertical to surface, which is represented by 

 upper margin of figure; x 15. 



4. Petrosia similis densissima. Section vertical to surface, which is represented by 



« 



u 



5. 



u 



« 



tt 



- upper margin of figure; x 15. 

 Dermal surface ; x 20, 



6. Gellius perforatus. Dermal surface; x 25. 







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