XII] ASTACUS 191 



FIG. 73. The Crayfish, Astacus fluviatilis, split into two by a median cut 

 extending along the mid- dorsal line and viewed from the side. 



1. Antennule. 2. Antenna. 3. Mandible. 4. Mouth. 5. Squame 

 of antenna. 6. Anus. 7. Telson. 8. Male genital opening. 



9. Chela. 10. First walking leg. 11. Second walking leg. 12. Third 

 walking leg. 13. Fourth walking leg. 14. The first abdominal 



appendages modified for copulation. 15. The second abdominal ap- 



pendages also modified for copulation. 16. The first swimmeret. 



17. The second swimmeret. 18. The third swimmeret. 19. The 

 tail fan last abdominal appendage. 20. Oesophagus. 21. "Stomach." 

 22. Mesenteron. 23. Cervical groove. 24. Intestine. 25. Cerebral 

 ganglion. 26. Nerve-collar. 27. Ventral nerve-cord. 28. Eye. 

 29. Heart. 30. Sternal artery (ventral part). 31. Abdominal 



artery. 32, 33. Sternal artery (sub-neural part). 34. Ophthalmic 

 artery. 35. Antennary artery. 36. Hepatic artery. 37. Testes. 

 38. Vas deferens. 39. Apodemes. 40. Kidney (glandular part). 



41. Kidney (bladder). 42. Kidney (opening). 



downwards and forwards on the lateral wall of the stomach to 

 meet and articulate with the pterocardiac ossicle. The lower 

 end of the zygocardiac ossicle is produced inwards into a comb-like 

 lateral tooth. The cardiac, pyloric, pterocardiac and zygocardiac 

 ossicles form a hexagonal frame of rods jointed together; so that 

 when the two ends are pulled apart the sides come inwards and 

 the lateral teeth meet and collide. But this is not all; to the 

 centre of the cardiac ossicle is articulated *a median sclerite, the 

 urocardiac, running directly backwards, and to the centre of the 

 pyloric is articulated a similar sclerite, the prepyloric, running 

 downwards into the groove dividing the urocardiac and pyloric 

 divisions of the stomach till it meets the urocardiac ossicle. 

 Articulating with both is the median tooth, a sclerite projecting 

 into the stomach. When the cardiac and pyloric sclerites are 

 dragged apart the urocardiac ossicle draws the median tooth forward 

 whilst the end of the prepyloric ossicle rotates it upwards and turns 

 the biting surface of the tooth forward and it comes into contact 

 with the lateral teeth. By the action therefore of the gastric 

 muscles the three teeth are caused to collide. The whole arrange- 

 ment is termed the gastric mill. 



The portion of the intestine lined by endoderm is termed the 

 mesenteron and it is not more than -^ inch long: it is produced 

 dorsally into a slight pouch called the caecum, whilst at each 

 side it is joined by the duct of a large gland called the liver. 

 This organ might be described as a pair of trees of tubes: it 

 consists on each side of a branched outgrowth of the mesenteron 

 the cells lining which are impregnated with pigment and produce 

 a juice which has a powerful digestive action on the food. It 



