MANUAL OF CONCHOLOGY. 



MONOGRAPH OF THE POLYPLACOPHORA. 



Class AMPHINEURA. 



Bilaterally symmetrical mollusks, the nervous system consist 

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

 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 com: 

 to Gastropoda and ^Amphineura are either features possess* 

 (hypothetical) primitive rnollusk, 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 Patell" 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 in 

 eating 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 y.rmitorm, 

 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) 



