FRESH-WATER MUSSEL. 



137 



the interlamellar spaces of the gills, and as the animals are usually 

 dioecious, it is probable that they are drawn in by the currents of water 

 inhaled by the female, and that the ova are impregnated after their ex- 

 trusion. The ova may be found in great abundance during the autumn 

 and winter months. They are nourished by a substance formed by the 

 epithelium of the spongy interlamellar junctions, and develope into the 

 Glochidium. This Glochidium is eventually set free from the parent. It 

 possesses a shell triangular in outline with an incurved tooth in the centre 

 of the free edge of the valve. The margin of each lobe of the embryonal 

 mantle has four sense organs, and a long embryonal byssus filament 

 protrudes from between the valves, by means of which the young animal 

 attaches itself when it quits the egg-membrane. It eventually fixes itself 

 to the fins, tail, &c., of fish, e. g. Stickleback, by means of its valve-teeth. 

 The epidermic cells of its host grow round and enclose it, and it then 

 undergoes a metamorphosis. The permanent mantle is formed, and the 

 rudimentary byssus gland, homologous with the byssus gland, e. g. of 

 Pinna, appears. The embryonal byssus gland of the Glochidium is not 

 homologous with the permanent gland as usually supposed. The nerve 

 ganglia and the otocysts are derived from the ectoderm. The single 

 adductor of the Glochidium disappears ; and the two adductors of the 

 adult are new formations, as is also the shell of the adult. 



According to Spengel, the supra-oesophageal ganglia of the Lamellibranch 

 represent the cerebral plus the pleural ganglia, i. e. portions of the parieto-splanchnic 

 ganglia so-called in the Snail, and independent ganglia in Limnaeus and many 

 other Gastropoda. It is a noteworthy point that in Lamellibranchiata there is 

 no connective between the pedal ganglia and the ganglia here called visceral. 

 Supposing that the latter ganglia were the homologues of the pleural and visceral 

 ganglia of e. g. Limnaeus, as ordinarily maintained, the absence of such a connective 

 would be an abnormality. It would not be, on the supposition that the pleural 

 ganglia are fused to the cerebral. The homology of the visceral ganglia with 

 the ganglia of the same name in Gastropoda, is also probable from their connection 

 with an osphradial apparatus. 



As to organs of special sense. Certain of the epidermic cells are modified 

 into tactile cells, continuous basally with nerve filaments and furnished at their 

 outer free ends with a bundle of fine projecting tactile hairs or setae. These 

 tactile cells are most plentiful on the papillae of the mantle edge, especially in 

 its siphonal region. They are more sparingly present round the edge of the 

 cloaca, on the edge of the fore part of the mantle, on the labial tentacles, inner 

 surface of mantle, and on the foot. 



Eyes are not present in Anodonta at any period of its existence, whether 

 larval or adult. Many larval Lamellibranchiata possess larval eyes at the base 

 of the velum close to the oesophagus. For the eyes of adult Lamellibranchiata, 

 see general account of the Class. 



An otocyst or auditory organ lies near to each pedal ganglion. It is sur- 



