The Arthropod a 



321 



the carbonic acid held in solution in the blood. From here it passes 

 by way of other gill channels into the branchio-cardiac sinuses ; thence 

 to the pericardial sinus into the heart. 



THE RESPIRATORY SYSTEM 



The crayfish, living in, and breathing through water, has branchial 

 chambers, which contain gills (Fig. 204, B) instead of lungs, to form its 

 respiratory system. These gills are pyramidal in shape and are thrown- 

 out into many flaps or lamellae closely packed together. Each gill has a 

 ventral and a dorsal vessel through which the blood from the body cavity 

 passes into the gills, spreading out through tiny capillaries into the 

 lamellae ( ). These capillaries are continuous with 



similar capillaries emptying into the dorsal vessel. 



Fig. 207. 



Semi-diagrammatic view of internal organs, and some limbs of right side of 

 a male Crayfish. Astacus fiuviatilis. 1. Antennule. 2. Antenna. 3. Mandible. 4. 

 Mouth. 5. Scale or squama of antenna, exopodite. 6. Anus. 7. Telson. 8. Opening 

 of vas deferens. 9. Chela. 10. 1st walking leg 11. 2nd walking leg. 12. 3rd 

 walking leg. 13. 4th walking leg. 14. 1st abdominal leg, modified. 15. 2nd 

 abdominal leg, slightly modified. 16. 3rd abdominal leg. 17. 4th abdominal leg. 18. 

 5th abdominal leg. 19. 6th abdominal leg, forming with telson the swimming 

 paddle. 20. Oesophagus. 21. Stomach. 22. Mesenteron, mid-gut. 23. Cervical 

 groove. 24. Intestine. 25. Cerebral ganglion. 26. Para-oesophageal cords. 27. 

 Ventral nerve-cord. 28. Eye. 29. Heart. 30. Sternal artery. 31. Dorsal abdom- 

 inal artery. 32. Ventral abdominal artery. 33. Ventral thoracic artery. 34. 

 Ophthalmic artery. 35. Antennary artery. 36. Hepatic artery. 37. Testis. 38. 

 Vas deferens. 39. Internal skeleton. 40. Green gland. 41. Bladder. 42. External 

 opening of green gland. (From Latter after Howes.) 



The venous blood in all parts of the body other than the gills, passes 

 through what is called an open sinus system, whereas in the gills them- 

 selves the anastomosing arch of the arterial and venous capillaries forms 

 a closed system. 



The thin-walled flaps of the gills are in contact with the water, which 

 is sent through the branchial chamber by the muscles of the scaphogna- 

 thite ( ), a sort of scoop consisting of the fused 



bract and exopodite of the second maxillae. This scoop bales the water 



