POLYPLACOPHOR \. 



VII 



and the latter corresponding clo>ely in .-rulptun- to the lateral areas' 



In some genera the muero i> near or at \\\> 



valve, and the posterior area is then reduced to a narrow 



altogether absent (see pi. OL', li<j>. 17, I*;. In position, ti. 



may be either anterior (pi. -^l liir. 1*3), or media,, < pi. 17, i 



ir (pi. .VJ, figs. 22, I*:!) ; and it may be either elevat^l | 

 fig. 9-J), or depressed (pi. 39, fig. 41), the last being sometime culled 

 a Hat or planate mucro. 



THE INNER LAYER OF Tin: VALVES (articulament i > i / 1 I- larger 

 than the tegmentum, projecting in front in two lobes called sntural 

 lamince, which are separated by a median bay, the j"ytl sinus. At 



the sides of the intermediate va! 

 and around the semicircle of the end 

 valves, most Chitons have projecting 

 plates called insertion plates, to whieh 

 the girdle is attached. These are 

 * commonly cut into teeth by transverse 

 slits. From the slits to the apex of 

 each valve inside, run slight grooves 

 and rows of pores, known as slit-rays; 

 often they are obliterated, but in forms 

 having a highly developed system of 

 sense-organs in the tegmentum, the 



Figs. 3, 4. "intermediate and P areS f the ^W* 9Q rve &S D6rve 



posterior valves of Ischnochiton foramina. The teeth are sometimes 

 conspicuus. finely cut or crenulated, (technically 



" pectinated ") between the slits ; and in some forms the edges of the 

 teeth are thickened outside, or " propped." Fig. 3 represents the 

 interior of the fourth valve of an Ischnochiton, showing the slits, 

 teeth, insertion plates, etc. 



The tegmentum is essentially cuticular in nature. It consi>' 

 a chitinous foundation substance, impregnated with salts of lime. 

 It is perforated at the surface by a multitude of minute, definitely 

 arranged pores of two sizes, called by Moseley, the larger //<"/<*/"/ 

 the smaller micropores (see pi. 52, fig. 28, showing two megalopores 

 and many micropores). Each megalopore leads into a cylindrical 

 chamber which is continued below into a wide canal, ffhich as it 

 penetrates deeper toward the plane of junction of tegmentum and 

 articulamentum, curves outward toward the girdle-margin of the 

 former. On reaching the plane of junction it joins a plexus of 



