610 THE MICROSCOPE. 



generally considered to be tubular. These lines exist in 

 every part of the shell sufficiently thick to present them, 

 and in certain positions, as in the end of the clawB, their 

 course gives them very much the appearance of dentine ; 

 but before they arrive at the cavity of the claw, they 

 degenerate into mere dots, and have nothing in the 

 character of tubuli. There can be little doubt that the 

 same may be said of the spaces existing between the longi- 

 tudinal portions of dentine. These latter spaces are in 

 principle analogous to the canaliculi of bone ; hence in 

 one form of bone, the cementum or crusta petrosa 

 caraliculi and dentinal canals sometimes exist together. 

 The dentinal canals are merely spaces of feeble or imperfect 

 cohesion, continued from the interglobular spaces, where 

 such exist, between longitudinal portions of coalesced den- 

 tine, to the pulp cavity. These passages, becoming a little 

 widened from the contraction of the part in drying, and 

 containing air, seem to have, under the microscope, especi- 

 ally if appearances depending upon distance are not suffi- 

 ciently allowed for, the appearance of tubes. It is, 

 however, incompatible with the function of tubes, as they 

 exist in other parts, that a system of such organs, intended 

 for the conveyance of secreted fluids, should be simple 

 prolongations of the interstices of the body, as are the 

 spaces between globular portions of dentine and the pulp- 

 cavity of a tooth, this latter, notwithstanding its size, 

 being only a cellular interval. The other structure is 

 distinctly tubular. It does not exist, excepting in the 

 tegument ary part of the shell. These tubes pass from the 

 external surface of the shell, through its substance, to its 

 deeper one. They have distinct parietes ; and their peri- 

 pheral extremities in different parts of the same shell 

 present very different forms. All along the convex border 

 of the claw each of these tubes projects beyond the surface, 

 where it becomes free, presenting generally a feathery 

 extremity, or sometimes it seems split up into small fibres 

 like a brush. Just at the point, and all along the biting 

 edge, these tubes are rather smaller and more numerous ; 

 but, as might have been expected, they have not the 

 feathery appendage. Notwithstanding, their ends are free 

 and prominent. In the tubes alluded to, no projecting 



