MANUAL OF CONCHOLOGY. 



MONOGRAPH OF THE POLYPLACOPHORA. 



Class AMPHINEURA. 



Bilaterally symmetrical mollusks, the nervous system consisting 

 of four (two lateral and two ventral or pedal) parallel cords, meeting 

 in a cerebral ganglion ; other ganglia feebly or not developed. 

 Anus posterior and median ; head without tentacles or eyes. 



The Amphineura or Isopleura constitute one of the five primary 

 divisions or Classes into which the Mollusk phylum divides ; and it 

 is, in many respects, the most primitive of all. This class has been 

 by most authors considered a sub-class or order of Gastropoda ; but 

 the fact should be distinctly recognized that the characters common 

 to Gastropoda and Amphineura are either features possessed by the 

 (hypothetical) primitive mollusk, or are peculiarities evolved in 

 response to similar habits of life. The Patellidce among Gastropoda 

 have been repeatedly compared to the Amphineurous family 

 Chitonidce ; but the branchial cordon of Patella is in no way homo- 

 logous with the gills (or ctenidia) of Chiton, and the nervous and 

 digestive systems are profoundly different. 



The class Amphineura is divisible into two Orders : 



Order I. POLYPLACOPHORA. Dorsal surface bearing eight imbri- 

 cating shelly plates ; head divided from the body ; gills numerous, 

 occupying a lateral groove on each side between the foot and the 

 encircling mantle ; foot adapted to creeping ; genital organs and 

 nephridia paired ; sexes distinct ; radula well developed. 



Order II. APLACOPHORA. Body subcylindrical or vermiform, 

 without shelly plates or valves ; foot rudimentary or aborted, the 

 mantle cavity being reduced to a ventral groove terminating in a 

 posterior cloaca containing the rudimentary gills, and the outlets of 

 anus and nephridia. 



(v) 



