ORDER ii APLACOPHORA POLYPLACOPHOifA 



Order 1. APLACOPHORA. 



Body vermiform with <> />/<//<// groove, tin- skin elsewhere beset with calcareous 

 spicules; no dorsal shell ij plates in tin' ml nit. 



This is a degenerate group, represented in the recent fauna by about a 

 dozen genera belonging to two families Chaetodermatidae and j\V.///r///W//. 

 Fossil remains are unknown. 



Order 2. POLYPLACOPHORA. Blainville. Chitons. 



Amphineura protected by a dorsal series of eight shell ij mires ami im <-/////>////// 

 girdle; with differentiated hem/, ami a ventral sole or foot adapted t<> wy/'/'v .- //;//> 

 numerous, occupying the groove between foot ami girdle ; radula present, heterodont ; 

 sexes separate. 



The external covering in the Polyplacophora, or Chitons, consists of eight 

 valves bound together by an encircling flexible band called the girdle. The 

 anterior or head-plate (Fig. 792, A, below) is invariably semicircular, with the 

 apex or mucro at the middle of the straight margin ; the six succeeding plates 

 are generally square (Fig. 793, below), with the apex posterior on the median 

 line ; and the posterior or tail- valve (Fig. 792, B) is semicircular or subcircular, 

 with apex varying in position from in front of the middle to the hind margin. 

 All of the plates are composed of two layers an outer porous layer, the 

 tegmentum, and an inner porcellanous one, the articulamentum. In most of the 

 lower Chitons these layers are coextensive and have smooth edges ; but in 

 the higher forms the articulamentum projects beyond the outer layer into the 

 substance of the girdle, in which it is firmly inserted. These projections at 

 the outer or peripheral margin are termed insertion plates. They are generally 

 slit or notched into so-called " teeth," which may be either smooth and sharp 

 along the edge, or crenulated (pectinated}. Insertion plates serve the function 

 of binding the valves firmly to the girdle. 



The anterior margin of each valve except the first one invariably shows 

 two projections of the articulamentum called sutural laminae (Figs. 792, B, 

 793), which pass under the rear margin of the next anterior valve, thus pre- 

 venting vertical displacement of the series. The tegumentum is traversed by 

 a multitude of fine canals which terminate at the surface idttunute sense 

 organs. The position of the latter in dry or fossil valves ; ~fl He as a fine 

 quincuncial punctation. In the highest Chitons a certain iHraoer of these 

 sense organs have become enlarged and modified into eyes, easily recognised 

 as pigmented dots in recent, and small pits in fossil specimens. I 



Polyplacophora make their appearance as early as the Ordovicfen ; they are 

 rare in the Silurian and Devonian, but somewhat more aburl|ant in the 

 Carboniferous. None of the Palaeozoic genera (Eoplacophora) are known to 

 continue into the Mesozoic, but are replaced by types more related to modern 

 Chitons (Mesoplacophora}. Members of the most specialised sub-order, Teleo- 

 placophoi'a, are first encountered in the Eocene, although they doubtless arose 

 earlier. About -twenty Palaeozoic, five or six Mesozoic, and fifty Tertiary 

 species have been described. Recent forms number several hundreds. 

 A good many species supposed to be Chitons have been based^fcui barnacle 

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