292 THE DERMAL SKELETON. 



to that outside the shell, or impregnated with a dark red or black substance. The 

 walls of these canals, which appear to have contained sense organs and mucous 

 glands, are very sharply defined. Bone lacunae do not open into them, and they 

 lead only at irregular intervals, if at all, into the other system of canals, or into the 

 cancellae. They appear to be uniformly distributed in the shell, throughout all 

 parts of the buckler. When the outermost layers are removed and the shell is 

 viewed as a semi-transparent object from its inner surface, the sensory canals are 

 seen to overlie the partition separating the cancellae. (Fig. 194.) 



The canals belonging to the outer set are smaller and much fainter than the 

 ones just described, and never contain pigment, or foreign materials derived from 

 the surrounding matrix. They form a horizontal polygonal mesh network of 

 slender irregular vascular canals, v.c., that open by larger ones into the summit of 

 the cancellae, c, and hence through the floor of the cancellae into the interior, i.p. 



exp 



' v 



FIG. 195. A, Tangential section of the outer layers of the exoskeleton of Tremataspis; B, outermost layer, in sur- 

 face view, more highly magnified; C, same in cross-section. 



Several strands of the arching horizontal canals lead into irregular spaces lying in 

 the center of the areas enclosed by the chimney pores. From the floor of these 

 spaces, numerous irregularly looped canals arise that project inward, forming ill- 

 defined tufts of vascular canals (Fig. 195, v.c.), on about the same level with the 

 large sensory canals. Some of these canals appear to open occasionally into the 

 floor of the sensory canals. 



From the roof and sides of all the outermost vascular canals, and from their 

 points of union with one another, arise numerous tapering vertical canals, the 

 osteo-dentinal canals. They are larger than the ordinary canaliculi, especially 

 at their proximal ends, and after expanding into a row of three or four overlying 

 lacunae, open into innumerable anastomosing canaliculae. The main axial canal 

 terminates in a single minute canaliculus, that runs through a faintly defined 

 cylinder to the outer surface. (Figs. 193, 195, e.) 



The layer of prisms with their axial pore canals make up the glassy outer 

 surface of the shell; the layer of osteo-dentinal canals forms the dentinal layer. 



The bone cells, or lacunae, lying in the spaces between the vascular and sen- 

 sory canals are smaller than the ones just described and are more like the typical 

 bone lacunae. They appear to lie between the concentric bone lamellae surround- 

 ing the canals. 



