XIII] 



ANODONTA 



307 



.16 



15- 



6- 



10 



row. From the upturned edges of these last it reaches the mantle 



and from this the auricle. Some blood is sent to the lobes 



of the mantle, as in the snail, and 



through the thin skin absorbs oxygen ; ? 



this blood is returned direct to the 



auricle without passing through the 



gill; from this fact it appears that 



the mantle lobe as well as the gill is 



a respiratory organ. 



In the nervous system the cere- 

 bral and pleural ganglia on each side 

 are generally regarded as coalesced, 

 but in Nucula a distinct pleural 

 ganglion has been observed on the 

 cerebro-visceral commissure anterior 

 to the pericardium. There is a long 

 visceral loop, ending in two closely 

 conjoined visceral ganglia, placed 

 beneath the posterior adductor (Fig. 

 139). On either side of these, just 

 where the axis of the ctenidium be- 

 comes free from the body, is a 

 thickened patch of yellow ectoderm 

 the osphradium. This is a pecu- 

 liar sense-organ, the function of 

 which, it is believed, is to test the 

 water passing over the gill as to 

 suitability for respiration. There 

 are a pair of large otocysts in 

 the foot. 



Mussels are male and female : 

 their reproductive organs are paired, 

 and consist on each side of a bunch 

 of tubes spreading through the foot. 

 The ducts are continuous with the 

 walls of the ovary or of the testis. 

 They open by slit-like orifices just 

 in front of the opening of the 

 nephridia on each side of the foot. 

 The spermatozoa are swept out by the water passing through the 

 dorsal siphon and are sucked in by the inhalant currents of female 

 individuals. The eggs wben cast out are detained between the two 



202 



-9 



FIG. 140. Dorsal view of Anodonta 

 mutabilis, with the upper wall 

 of the pericardium removed to 

 show the heart x about 1. After 

 Hatschek and Cori. 



1. Foot. 2. Anterior adductor 

 muscle. 3. Protractor muscle. 

 4. Depressor muscles. 5. Pos- 

 terior retractor muscle. 6. 

 Posterior adductor muscle. 7. 

 Dorsal siphon. 8. Ventral 



siphon. 9. Anus. 10. Split 

 between left and right mantle 

 lobes through which larvae 

 at times leave the epibranchial 

 chamber. 11. Keber's organ. 



12. Bectum traversing ventricle. 



13. Internal opening of organ 

 of Bojanus. 14. Ventricle. 

 15. Left auricle. 16. Anterior 

 retractor muscle. 



