70 



ESSENTIALS OF ZOOLOGY 



of the former runs round the mantle and returns the blood 

 to the external efferent branchial trunk both anteriorly and 

 posteriorly ; the blood is arterialised, for it has been in con- 

 tact with the water of the mantle cavity. The venae cavae 

 lie under the pericardial cavity and send the blood to sinuses 

 surrounding the glandular part of the kidney. Efferent 

 renal vessels carry the blood from the kidney to the gills. 

 The collecting vessel or afferent branchial trunk runs along 

 the attachment of the outer and inner gills on each side, and 

 the blood is carried by afferent branchial veins into the sinuses 



of the gills. In the gills the 

 blood passes through spaces 

 containing lymphoid tissue and 

 through the blood spaces of 

 the filaments. The blood is 

 returned from the gills by 

 efferent branchial veins which 

 culminate in the efferent bran- 



Skel. bar 



. vessel 



Aff. vessel 



Skel bar 



FIG. 34. Anodonta. Transverse 

 section of external gill, showing 

 an intralamellar junction. 



chial trunks internal and 

 external. The external trunk 

 receives veins from the peri- 

 cardial gland on each side. 

 The efferent branchial veins 

 are joined, as has been said, 



by the pallial veins anteriorly and posteriorly, and the 

 combined arterial blood is then sent into the auricle on each 

 side of the body, which also receives the blood from the 

 internal efferent branchial trunk. 



Nervous System. The nerve ganglia in the fresh state are 

 of an orange colour, and the nerves associated with them are 

 white and fine. The cerebral ganglia may be seen without 

 dissection just above the mouth and between the anterior 

 adductor and the protractor muscles. The pair of pedal 

 ganglia are situated close together medianly in the foot, just 

 above the muscular part of the foot. They may be exposed 

 by splitting the foot longitudinally or by dissection, following 

 the course of the cerebro-pedal commissure. The pair of 

 visceral ganglia lie close together on the under side of the 

 posterior adductor. The cerebral ganglia are connected by 



