280 HANDBOOK OF INYERTEBEATE ZOOLOGY. 



Fig. 144. — Anodonta cygnea, seen from the left side. The mantle 

 and gills of the left side, the labial palpi, part of the pericardium and 

 part of the organ of Bojanus have been removed. (From KoUeston, 

 Forms of Animal Life, Plate Y. ) 



a. Right mantle lobe. a'. Branchial siphon, o". Dorsal edge of 

 mantle. 6. Foot. c. Gills, d. Anterior adductor, e. Posterior adduc- 

 tor. /. Posterior foot-retractor, g. Foot-protractor, h. Anterior foot- 

 retractor, i. Cerebral ganglia, j. Cerebro-pedal commissure. j\ Audi- 

 tory organs. k. Cerebro-visceral commissure. I. Parieto-splanchnic 

 ganglia, m. Rectum, n. Heart, o. Pericardium, p. External opening 

 of organ of Bojanus. q. Channel of communication between its glandu- 

 lar and non-glandular portions. )•. Opening between glandular portions, 

 s. Glandular portion, t. Reproductive orifice. 



labial nerves which pass to the palpi, and to three commis- 

 sures. 



(i.) One of these is the cerebro-visceral commissure, 

 which has just been traced. 



(ii.) Another is the cerebral commissure, which passes 

 in front of and dorsal to the mouth, and joins the two 

 cerebral ganglia to each other. 



(iii.) The third is the cerebro-pedal commissure (Figs. 

 143 and 144, /), which runs downwards along the anterior 

 edge of the abdomen, under the muscles. 



3. In the foot these commissures bend backwards, and 

 join the pair of pedal ganglia (Fig. 143,^,^). These two 

 ganglia are fused with each other on the median line, and 

 they are embedded in the' muscles of the foot in such a 

 way that they cannot be found without careful dissection. 

 They are at some distance from the outer surface, and 

 very near the inner or abdominal surface of the foot. 

 They give rise to a number of nerves which pass to the 

 muscles of the foot. 



4. If possible find a very young specimen of Unio or 

 Anodonta, one less than quarter of an inch long, and 

 having cut out the foot, place it upon a glass slide, and 



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