ORDER II APLACOPHORA—POLY PLACOPHORA 433 
Order 1. APLACOPHORA. 
Body vermiform with a ventral groove, the skin elsewhere beset with calcareous 
spicules ; no dorsal shelly plates in the adult. 
This is a degenerate group, represented in the recent fauna by about a 
dozen genera belonging to two families—Chaetodermatidae and Neomeniidae. 
Fossil remains are unknown. 
Order 2. POLYPLACOPHORA. Blainville. Chitons. 
Amphineura protected by a dorsal series of eight shelly valves and an encircling 
girdle ; with differentiated head, and a ventral sole or foot adapted to creeping ; gills 
numerous, occupying the groove between foot and 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, 4, 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, 6) is semicircular or subcireular, 
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 in minute sense 
organs. The position of the latter in dry or fossil valves is visible as a fine 
quincuncial punctation. In the highest Chitons a certain number of these 
sense organs have become enlarged and modified into eyes, easily recognised 
as pigmented dots in recent, and small pits in fossil specimens. 
Polyplacophora make their appearance as early as the Ordovician ; they are 
rare in the Silurian and Deyonian, but somewhat more abundant in the 
Carboniferous. None of the Palaeozoic genera (Hoplacophora) 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- 
placophora, ave 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 upon barnacle 
VOL. I 2F 
