CH. II] THE CRAYFISH 51 



Fig. 7. Semi-diagrammatic view of internal organs, and some limbs of 



right side of a male crayfish, Astacus fluviatilis x 1. Partly from 



Howes. 

 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. (Esophagus. 21. Stomach. 



22. Mesenteron, mid-gut. 23. Cervical groove. 24. Intestine. 



25. Cerebral ganglion. 26. Para-cesophageal cords. 27. Ventral 



nerve- cord. 28. Eye. 29. Heart. 30. Sternal artery. 



31. Supra-intestinal artery. 32. Sub-intestinal artery in abdomen. 



33. Sub-intestinal artery in thorax. 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. 



seized if carefully insinuated into the mouth of the 

 burrow, and may be conveniently used as a bait by the 

 amateur crayfish hunter. 



When advancing in search of food the animal moves 

 stealthily forward, walking slowly by means of its thoracic 

 legs assisted by the paddling action of the abdominal 

 swimmerets. In walking, the first three pairs of legs pull 

 and the fourth pair pushes. Their order of movement is 

 as follows : the first on the right and the third on the left 

 side move together, next the third right and first left, 

 then the second right and fourth left, and lastly the fourth 

 right and second left. The body is thus always supported 

 by six feet while the other two are advancing 1 . The 

 nippers (Chelce) are usually carried clear of the bottom, 

 and partly flexed so as to be capable of quick movement 

 forward. The third maxillipedes search the bottom in a 

 tentative manner and appear to possess a very delicate 



1 List, Morph. Jahrb. xxn. 



42 



