44 COMPARATIVE ANATOMY. 



tioned throughout its entire length. The ' labial ' ganglion is 

 seen lying on the anterior retractor muscle of the foot, and 

 immediately below the anterior adductor. Three commissural 

 cords, under each of which a slip of blue paper has been placed 

 pass off from it. The one which is seen passing vertically 

 downwards along the base of the inner gill and through the 

 organ of Bojanus, connects the labial with the parieto- 

 splanchnic or branchial ganglion, which is seen lying on the 

 posterior adductor. A little way to the left may be seen the 

 terminal portion of the corresponding commissural cord of the 

 opposite side. A slip of blue paper has been placed beneath 

 it. A second cord is seen passing obliquely to the left, into the 

 substance of the foot, to join the pedal ganglion. The third 

 cord passes upwards towards the anterior adductor, and 

 connects the labial "ganglion with its fellow of the opposite 

 side. From the pedal ganglion several nerves may be seen ra- 

 diating to supply the muscular foot. A black bristle has been 

 placed beneath a bundle of them. The two parieto-splanchnic 

 ganglia of the two sides of the body are closely apposed, so as 

 to form a transversely oblong mass. Nerves from these ganglia 

 may be noticed passing to the mantle and inner gills. 



71. Two Oysters (Ostrea edulis). 



The upper specimen shews the single adductor muscle; the 

 lower, the hinge-ligament, which causes the valves to open. 

 Each valve is composed of a series of concentric calcareous 

 laminae, overlapping each other by their free margins. 



72. A specimen of PJiolas dactylus, to illustrate the ar- 

 rangement of the mantle in a Siphonate Gasteropod. 



A red rod has been placed in the exhalent orifice, and a 

 blue rod in the inhalent. 



The valves of the shell have been partially opened, so as to 

 shew the union of the ventral margins of the mantle. In front 

 the mantle-lobes are widely separated, so as to allow of the 

 protrusion of the muscular foot between them. In front of the 

 foot they are again united. Inferiorly the mantle-lobes are 

 prolonged downwards into a cylindrical tube, divided into two 

 canals by a median attachment. These canals are the exhalent 

 and inhalent orifices. 



