LAMELLIBRANCHIA TA. 



331 



FIG. 256. Mytilus edulis attached by its byssus to a piece 

 of wood (after Meyer and Moebius). 



linear club-shaped or cylindrical foot (Solen, Solenomya\ and move 

 by rapidly retracting it and ejecting water through the siphons. 

 Many use the foot for burying themselves in mud ; others bore into 

 wood (Teredo}, or hard rock (Pholas, Lithodomus, Saxicfiva, etc.), 

 for which purpose 

 they push them- 

 selves against the 

 rock with their 

 short blunt foot, 

 and use the hard 

 and often serrated 

 edge of their shell 

 as a ' grater, giving 

 it a rotatory move- 

 ment. According to 

 Hancock, the foot 

 and edge of the 

 mantle at the an- 

 terior edge of the gaping shell are beset with silicious crystals, and 

 effect the excavation of the rock after the manner of a file. 



The protraction of the foot is due to its turgescence by blood ; its retraction 

 to the retractor muscles. An aquiferous pore by which the vascular system 

 communicates with the exterior does not exist. 



The byssus-gland opens by a pore in the middle line of the foot. It is 

 well-developed in the adult in Anomia, Area, Mytilus, Avicula, Pecten, 

 Saxicava, Lyonsia, Tridacna, Dreissena, etc. In Anomia the byssus passes 

 through a hole in the right valve, and is calcified. 



The nervous system presents three pairs of ganglia, the cerebral 

 (supra-oesophageal), pedal (sub-oesophageal), and visceral ganglia 

 (Fig. 257). In Nucula (Fig. 258) there is, in addition, close to 

 the cerebral a pair of pleural ganglia; these are connected with 

 the pedal by a pleuro-pedal connective which, however, joins the 

 cerebro-pedal connective before it reaches the pedal. In Solenomya, 

 in which there is also a pleural ganglion, the pleuro-pedal con- 

 nective is fused with the cerebro-pedal throughout almost its whole 

 extent. In all other Lamellibranchs a pleural ganglion is not 

 present as a distinct ganglion, but is in all probability fused with 

 the cerebral, which must, therefore, be regarded as a cerebro-pleural 

 ganglion. The visceral ganglia are on the hinder part of the 

 visceral loop ; they are usually placed on the ventral side of the 

 posterior adductor muscle. 



