VI 



POLYPLACOPHORA. 



Order POLYPLACOPHORA Blainville. 



The further division of this order into families and genera will 

 be found on page xxiv. 



The following notes on the morphology of Chitons are mainly 

 restricted to features important from a phylogenetic or taxonomic 

 standpoint ; the primary object of this work being, of course, the 

 establishment of a natural system of the Order. 



THE SHELL. 



The shell in Chitons consists of eight imbricating pieces or valves, 

 bound together by a leathery girdle of connective tissue. The valves, 

 when freed of the girdle by soaking a few hours in water, are seen to 

 be of three forms : the anterior, (or " head valve ") semicircular in 

 outline its apex elevated; the intermediate (sometimes called 

 " central " or " median ") valves, squarish in shape, and the poste- 

 rior or " tail valve," which is like the intermediate valves with the 

 addition of a sloping surface behind the apex or mucro. In struct- 

 ure, the valves are composed of two layers, generally quite different in 

 color and texture ; an older layer, called by Middendorff the tegmeii- 

 tum, and an inner, the articulamentum. 



THE SURFACE OF THE VALVES (tegmentum) is divided in nearly 

 all Chitons into clearly defined or indistinct areas. The interme- 



diate valves (fig. 1) are divided into 

 lateral areas and a central area; the 

 latter being subdivided into a dorsal or 

 jugal tract, extending along the ridge 

 of the valve, and two pleura orpleural 

 tracts, occupying the side-slopes in 

 front of the diagonal line or rib. In 

 some forms (fig. 2) the diagonal line is 

 obliterated, the lateral areas and the 

 pleural tracts being united into a single 

 uniformly sculptured expanse, the 

 Fig. 1. Intermediate (fourth) latero-pteural area, on each side; the 

 valve of hchnochiton merfensii, 1, dorsal or jugal area remaining distinct. 

 2, 3, central area (subdivided into ^ modincation is characteristic of 

 1 jugal tract, and 2, 3, pleural L u 1 i-nu^ A 



tracts) ; 4, 5, lateral areas. ne g reat P b y lum f Chitons. As a rule 



Fig. 2. Intermediate valve of tne head-valve is sculptured all over 

 Acanthochites defilippu, showing /. like the lateral areas. The posterior 

 PL, insertion plate, and S. Z., valve (fig. 4) is divided into a central 

 sutural laminae. an( j a p OS i er i or area ; the former being 



precisely similar to the area so named in the intermediate valves 



