XIII] 



ANODONTA 



301 



and it is by means of them that, when danger threatens, the animal 

 closes the valves and shelters foot, gills and body, within. Just 

 behind the anterior adductor is a pair of small muscles running into 

 the foot, and these are the anterior retractors of the foot. A 

 similar pair, the posterior retractors, are found just in front of the 

 posterior adductor, and by the combined action of the four re- 

 tractors the shell is drawn forward, the foot being (relatively) fixed 



12 



11 . 



10 



Fio. 138. A. Diagrammatic section through Anodonta to show the circulation 

 of the blood. B. Section through Anodonta near the posterior edge of 

 the foot. From Howes. 



A. 1. Bight auricle. 2. Ventricle. 3. Keber's organ. 4. Vena cava. 

 5. Efferent branchial trunk. 6. Efferent pallial vessel. 7. Efferent 

 branchial vessel. 8. Branchiae. 9. Afferent branchial vessel. 

 10. Efferent renal vessel. 11. Afferent branchial trunk. 12. Afferent 

 renal vessel. 13. Eectum. 



B. 1. Eight auricle. 2. Epibranchial chamber. 3. Ventricle. 4. Vena 

 cava. 5. Non-glandular part of the kidney. 6. Glandular part of 

 the kidney. 7. Intestine in foot. 8. Pericardium. 9. Shell. 

 10. Ligament of shell. 



in the mud (Figs. 137, 140). The foot is thrust forth by the forcing 

 of blood into it, through the contraction of the muscles which 

 underlie the skin in various parts of the body. The protractors 

 (of the foot) enable the animal to move backwards when necessary. 

 They are a pair of muscles inserted in the shell below the anterior 

 adductor and their fibres proceed upwards to join the upper part of 

 the foot. A 'small group of muscles running from the mantle to 



