20 



ARKIV FÖR ZOOLOGI. BAND 8. N:0 10. 



observed of. The air-sac is in its primitive form only an 

 extended stomach. In this family it is highly developed, 



■Epidermis 



Exteriör layer of 

 corium 



•Spine, hornogene- 

 ous part 



•Interiör layer of 

 corium 



Spine, fibrillated 

 part 



Fig. H. — Spheroides testudineus L. — Young speeimen. Diagram of ver- 

 tical longitudinal section of the skin. 



separated by a sphincter muscle from the rest of the stomach 

 and united with the ventral body wall. The air-sac is in- 

 flated with air. It then presses upon the skin of the ventral 



