450 THE ANIMAL KINGDOM. 



Lamellibranckiata, is co-extensive with the foot in some Gastropoda, 

 in others developed along a dorso-pedal axis, but with rare exceptions 

 it is in this Class, whether large or small, spirally twisted from behind 

 forwards along the right side of the animal. There is a consequent dis- 

 placement of the ctenidia, viscera, and the nephridial and anal apertures. 

 In Cephalopoda and Pteropoda it is also much enlarged but along an 

 oblique axis, which is directed backwards between the dorso-pedal and 

 antero-posterior axes. A spiral torsion of this dome is observed only in 

 one family of Pteropoda, the Limacinidae ; but whilst the Cephalopoda have 

 retained bilateral symmetry it is almost certain that the Pteropoda 

 have acquired it. A head is well-developed and distinct in Gastropoda 

 Anisopleura and Gymnosomatous Pteropoda. It is obscured in Cephala- 

 poda by the growth round it of the foot. In other Mollusca it is more or 

 less rudimentary. 



The respiratory organs are ctenidia, or gills, external processes of 

 the body. In some instances, Pteropoda, a few Gastropoda, respiration is 

 carried on entirely by the surface of the skin. There can be little doubt, 

 however, that such forms have lost the gills. A ctenidium consists essen- 

 tially of an axis containing an afferent and efferent blood-vessel and 

 giving support on either side to a series of vascular lamellae or processes, 

 the surface of which is ciliated except in Cephalopoda. The ctenidial 

 axis remains either free, e. g. Nautilus ; or partly free, partly attached to 

 the side of the sub-pallial space ; or attached throughout its whole length, 

 e. g. Dibranchiate Cephalopoda. The respiratory processes it bears 

 generally assume a most complicated structure in Lamellibranchiata. 

 The ctenidia are always lodged in the sub-pallial space, which may be 

 much enlarged in the region where they lie forming a branchial cavity. 

 In the Polyplacophora there are a number of them, but in all other 

 Mollusca they are typically two, one on each side of the body. In the 

 majority of Gastropoda fat primitive left ctenidium is aborted. Occasionally 

 both are aborted, and then respiration is carried on either by secondarily 

 developed vascular ridges lodged in the sub-pallial space, as in the Limpet, 

 or by the part of the mantle which forms the roof of the branchial cavity, 

 as in Pulmonate Gastropoda, or by the surface of the body. 



The nervous system consists typically of a pair of cerebral ganglia, 

 placed above the oesophagus, and two pair, pedal and pleural, placed 

 below it. The two members of each pair of ganglia cerebral, pedal and 

 pleural are united to one another by nerve-commissures : the cerebral 

 to the pedal and pleural, as well as the pedal to the pleural, by nerve - 

 connectives. Commissures and connectives vary in length, and consequently 

 there is not only a greater or less degree, but also a varying mode of 

 concentration of the ganglia among themselves. The cerebral ganglia 

 supply the head, the organs of touch, the otocysts (except in Lamelli- 



