BUB-KINGDOM vi MOLLUSCA 345 



intestinal canal, ;iml anus; a closed, Imt partly lacunary circulation, assisted 

 by a heart with one or more auricles, and containing a usually colourless 

 body fluid or haemolymph ; a nervous system with at least three pairs of 

 --.-uiglia connected by commissures; sexual reproduction by eggs and sper- 

 matozoa ; audition and equilibration provided for by otocysts ; respiration 

 by ctenidial or secondary gills, or by the tegumentary surface, which may be 

 invaginated to form a pulmonary sac ; locomotion by a muscular organ called 

 the foot, or by special parapodial structures ; the organs normally paired, 

 and protected by a sac-like integument called the mantle ; and the visceral 

 sac having a tendency toward torsion, so as to become usually asymmetrical. 

 Sexually Mollusks are* usually dioecious ; or, if monoecious, incapable of self- 

 fertilisation. 



Owing to the homogeneity of the group, its division into classes has been 

 attended with some differences of opinion, depending upon the point of view, 

 the anatomist laying more stress upon certain groups of characters, and the 

 morphologist upon others. From a general standpoint, the Mollusca are 

 readily divisible into four classes, as follows : Pelecypods, Scaphopods, 

 Gastropods, and Cephalopods. The first of these is well marked off from the 

 rest by the absence of a distinct head and of a radula, and the two groups 

 have been contrasted as Glossophora (or Cephalophora) and Aglossa (or Lipo- 

 cephala). Eecent researches tend to show that the Pelecypoda form a degenerate 

 group, more nearly related to the Protogastropod than is the latter to the 

 Protocephalopod ; and that in any general arrangement, the Cephalopods are 

 rather to be contrasted with the three other groups combined. 



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