. 174 



The nervous system consists of three pairs of ganglia : 

 cerebro-pleural ganglia, one at each side of the mouth, 

 under the skin, pedal ganglia in the middle of the " foot," 

 just within the visceral mass, and visceral ganglia on the 

 ventral surface of the posterior adductor muscle. The 

 pedal and visceral ganglia are connected by commissures 

 to the cerebral ganglia. There are two otocysts hcsi'lo the 

 pedal ganglia. Eyes are absent. 



Explain the Structure of the Gills of Anodonta. How (Jo 

 the Gills of Mytilus differ from thoxe of Aod(//f/ ? 



The gills (ctenidia or branchia), two a i each side of the 

 body, are long and plate-like, and their surfaces have 

 slight vertical ridges indicating the numerous gill filaments 

 of which they are composed. Each filament somewhat 

 resembles the letter V, and so each gill is a double plate 

 consisting of an inner and outer lamella. The outer i;;ill 

 has the outer lamella attached dorsally to the mantle 

 along its whole length ; the inner lamella and the outer 

 lamella of the inner gill are united dorsally and together 

 attached along the visceral mass. The inner lamella of 

 the inner gill is also attached dorsally (its front portion 

 only) to the visceral mass ; its middle portion is free, 

 and its hind portion, extending beyond the visceral mass, 

 is united with the corresponding lamella of the inner gill 

 of the other side of the body. All the gills extend beyond 

 the visceral mass and below the posterior adductor to a 

 point behind that muscle ; and in that region their lamellae 

 being all united dorsally, a partition is thus formed dividing 

 the mantle-cavity into an upper supra-branchial and a 

 lower infra- branchial chamber. The upper chamber is 

 continuous with the passages above and between the gill 

 lamellae throughout the body. The slender gill filaments 

 are ciliated and have supporting chitinous rods, and they 

 are joined together by horizontal strands of tissue (inter- 

 filamentar junctions) ; their V-portions are similarly 

 connected, and thus the space between the lamellae of 

 a gill is traversed by interlamellar junctions and divided 

 up into numerous water-tubes. 



