GEOWTH OF THE TOOTH OF ECHINUS. 
389 
In the anatomical details of this memoir I shall have occasion hereafter to allude 
to Professor "WiLLiAiiso^f’s paper, and I will only now add that its important new 
points are — the distinct assertion that the tissues of the tooth, cell-like as they are when 
matured, are formed without cells, being (to use his own expression) produced “ by the 
calcification of the intercellular fluid;” and, secondly, the discovery of the little cal- 
careous discs by which the elementary parts of the tooth are united together ; for though 
I differ horn him entirely as to the anatomical relations of these bodies, he has certainly 
given the first published account of them *. 
It is not my intention to enter upon any description of the many structures concerned 
in the complicated tooth-apparatus of the Ecliinus ; I confine myself strictly to the 
structure and elaboration of the teeth themselves ; the other elementary parts of the 
apparatus being already described in works treating of the comparative anatomy of 
Invertebrata. I may, however, premise that the internal growing extremity of the tooth 
is enclosed in a little membranous shut sac, containing numerous pigmentary cells and 
a clear aqueous fluid. The cells do not in any way enter into the formation of the tooth- 
structm’es which are free in the fluid of the sac. 
The tooth of the Echinus is a slender calcareous rod, of a deflnite and constant form. 
It may be divided for the convenience of description (though the division is artiflcial) 
into the shaft and ’‘'■plumule^,'' the shaft being the consolidated structure, the plumule 
the soft gro'wing extremity. The shaft of the tooth is nearly straight, being very shghtly 
curved, the concavity looking (when the tooth is in its natural position) towards the 
alimentary canal. The plumule is more strongly bent, and forms a little ringlet at the 
internal extremity of the organ (see Plate YI. flg. 1). In viewing the flgure referred to, 
it will be seen that, in this lateral aspect, the plumule is much narrower than the shaft 
of the tooth ; and further, that in tracing the plumule into the shaft it is found to be 
continuous only with that portion of the latter which forms the convex part of the tooth. 
It is important to bear this in mind, as it assists in understanding what part of the 
mature tooth is composed of those structural elements which constitute the plumule, 
and ha'ving this knowledge the observer can more readily associate the supplemental 
elements with their own position in the organ when completely elaborated. The 
general outline of the body of the tooth is displayed by a transverse section. Before 
considering this, however, there is one point which is best seen in the lateral view of 
the entire organ and its shghtly magnifled vertical section, namely, the apex or external 
point of the tooth. Both Quekett and Williasison have flgured and described the 
apex of the tooth as consisting of a sharp cuttmg external edge — chisel-shaped, like the 
incisor of a rodent. Such a description, however, is anatomically incorrect, and such a 
form is physically impossible. Truly the apex of the Echinus-tooth is retained in a 
definite form by the friction wear of elementary parts of difierent density, as a rodent’s 
* Unless indeed Valentin’s figure (tab. G, fig. 105 a) refers to them : if so, their regular linear arrange- 
ment, as represented by him, is incorrect. 
t The term “plumule” is Valentin’s. 
3 H 2 
