482 MOLLUSC A 



Key to the classes of Mollusca: 



Ux No distinct head present. 



61 Shell when present not bivalve. 



Cj Body either naked or with shell consisting of 8 plates 1. Ampiiineura 



C3 Cylindrical shell present 2. Scapiiopoda 



62 Bivalve shell present 4. Pelecypgda 



flj Distinct head present. 



&i Head with 1 or 2 pairs of tentacles; shell coiled or conic. .3. Gastropoda 

 6a Head with long arms 5. Cephalopoda 



Class 1. AMPHINEURA.* 



Bilaterally symmetrical mollusks, worm-like in appearance and with 

 usually an inconspicuous head. The mouth is at the forward and the anus 

 at the hinder end of the body, and on either side are the gills and the 

 nephridial pores. The mantle is not extensively developed but consists 

 of a slight fold extending from the sides and the hinder end of the body, 

 enclosing a narrow mantle cavity (Fig. 746, 5). The integument of the 

 back is thick and contains calcareous sjoicules or is covered with scale- 

 like plates. A radula is usually present. The nervous system (Fig. 745) 

 is very primitive in form, consisting of an oesophageal ring and four 

 longitudinal nerves, two pedal, which innervate the foot, and two pallial, 

 which pass along the mantle furrow; these may or may not contain 

 definite ganglia. 



History.— The class Amphineura was formed in 1877 by von Ihering, 

 who maintained, as Huxley and Leuekart had already done, that Chiton 

 was related to Chcetoderma, which, untii then, had been placed among the 

 Gepliyrea, and to Neomenia, whose position in the system had been very 

 uncertain, but he placed the new class among the worms. Chiton was first 

 studied by Cuvier, although it had been long known, who placed it beside 

 Patella among the Prosohrancliiata, where it remained almost down to the 

 present day. Gegenbaur in 1878 erected the group Solenogastres, which he 

 placed among the worms. In 1881 Spengel brought the Amphineura of 

 von Ihering among the Mollusca, where they have since remained as their 

 most primitive representatives. The modem arrangement of the group 

 is due largely to Pilsbry and Simroth. 



The Amphineura are all marine, living in most cases on the bottom, 

 near the shore, or in moderate depths. The class contains two orders, 

 with about 630 species. 



Key to the orders of Amphineura: 



Oi No shell present 1. Aplacophora 



Oa Shell present 2. Polyplacophora 



* "Amphineura," by H. Simroth, Bronn's Klass. u. Ord., Vol. 3, 1892. 



