14:4: THE MICROSCOPE AND ITS REVELATIONS. 



hair-like processes attached to the shell of Clypeaster, are composed of 

 the like regularly-reticulated substance; and these are very beautiful 

 objects for the lower powers of the Microscope, when laid upon a black 

 ground and examined by reflected light without any further preparation. 

 It is interesting also to find that the same structure presents itself in 

 the curious Pedicellarica (forceps-like bodies mounted on long stalks), 

 which are found on the surface of many Echinida, and the nature of 

 which was formerly a source of much perplexity to Naturalists, some 

 having maintained that they are parasites, whilst others considered them 

 as proper appendages of the Echinus itself. The complete conformity 

 which exists between the structure of their skeleton and that of the 

 animal to which they are attached, removes all doubt of their being 

 truly appendages to it, as observation of their actions in the living state 

 would indicate. 



535. Another example of . the same structure is found in the peculiar 

 framework of plates which surrounds the interior of the oral orifice of 

 the shell, and which includes the five teeth that may often be seen 

 projecting externally through that orifice; the whole forming what is 

 known as the ' lantern of Aristotle.' The texture of the plates or jaws 

 resembles that of the shell in every respect, save that the network is 

 more open; but that of the teeth differs from it so widely, as to have 

 been likened to that of the bone and dentine of Vertebrate animals. 

 The careful investigations of Mr. James Salter, 1 however, have fully 

 demonstrated that the appearances which have suggested this compari- 

 son are to be otherwise explained; the plan of structure of the tooth 

 being essentially the same as that of the shell, although greatly modified 

 in its working-out. The complete tooth has somewhat the form of thai 

 of the front tooth of a Eodent; save that its concave side is strengthened 

 by a projecting ' keel,' so that a transverse section of the tooth presents 

 the form of a j_. This keel is composed of cylindrical rods of carbonate 

 of lime, having club-shaped extremities lying obliquely to the axis of the 

 tooth (Fig. 371, A, d)'j these rods do not adhere very firmly together, so 

 that it is difficult to keep them in their places in making sections of 

 the part. The convex surface of the tooth (c, c, c) is covered with a 

 firmer layer, which has received the name of 'enamel; ' this is composed 

 of shorter rods, also obliquely arranged, but having a much more inti- 

 mate mutual adhesion than we find among the rods of the keel. The 

 principal part of the substance of the tooth (A, b) is made-up of what 

 may be called the ' primary plates; ' these are triangular plates of 

 calcareous shell-substance, arranged in two series (as shown at B), and 

 constituting a sort of framework with which the other parts to be 

 presently described become connected. These plates may be seen by 

 examining the growing base of an adult tooth that has been preserved 

 with its attached soft parts in alcohol, or (which is preferable) by exam- 

 ining the base of the tooth of a fresh specimen, the minuter the better. 

 The lengthening of the tooth below, as it is worn-away above, is mainly 

 affected by the successive addition of new ' primary plates.' To the 

 outer edge of the primary plates, at some little distance from the base, 

 we find attached a set of lappet-like appendages, which are formed of 

 similar plates of calcareous shell-substance, and are denominated by Mr. 

 Salter ( secondary plates.' Another set of appendages termed 'flabelli- 



'See his Memoir ' On the Structure and Growth of the Tooth of Echinus,' in. 

 " Philos. Transact." for 1861, p. 387. 



