212 MESSRS. HANCOCK AND EMBLETON ON THE ANATOMY OF DORIS. 
bottom of the follicle above described. The advance upwards of the teeth is assisted, 
in all probability, by the secretion of some matter supplementary to the dentigerous 
membrane by the surface of the tube in which it is contained. This surface is formed 
by a fine membrane, which in the follicle below is visible enough, but in the more 
muscular part of the tube above is not so easily demonstrated. This membrane, 
which is continuously spread over the whole of the buccal cavity, is analogous to the 
mucous membrane of the mouth of higher animals ; the tube formed by it, and the 
pouch which terminates it, compose a follicle, the secretion of which is the dentige- 
rous membrane beset with ranks of denticles, the analogues of epithelium and teeth. 
The inner of the two parallel membranes, and the flocculent matter in which the 
teeth are at first imbedded, gradually become detached and fall off, leaving the teeth 
bare as they reach their field of action. 
The mode of growth of the spiny tongue of Doris is thus evidently quite analogous 
to the growth and advance of the teeth of Rays, Sharks, &c., or of the hoofs or nails 
of Mammalia. 
In D. tiiherciilata the number of rows of denticles is forty-three or forty-four, eighteen 
of which are situated above the transverse septum or top of the follicle, and each con- 
tains 140 denticles; in this species therefore there are 6160 little teeth. These are 
not all of the same size, but diminish from near the ends of each row towards the 
centre, where the row is slightly interrupted, there being no central tooth*. The five 
or six external denticles also rapidly decrease in size towards the end of the row. 
Each denticle'l' is expanded broadly at the base, and is well arched backwards and 
sharp pointed, somewhat resembling the spines of the dog-rose. 
In D. verrucosa and D. tuherculata, Verany, the buccal organ and tongue are con- 
structed after the above model. 
In D. Johnstoni the buccal and lingual apparatus follow the same arrangement, the 
rows of denticles being twenty-four in number, of which eleven are used in prehen- 
sion. There are about fifty denticles to each row ; the five outermost at the ends of 
the rows are much attenuated, being nearly linear:]:. 
In D. coccinea the above parts are after the same type ; the external attenuated 
teeth of each row being much more numerous than in the last-named species. 
In D. repanda we have the buccal mass and the tongue resembling’ those of 
D. tuherculata. There are sixty-eight rows of toothlets, and thirty-six toothlets in 
each row, but they are of more elaborate character, having their edges serrated : in 
each row there is a centre toothlet bearing four cusps §. Three or four of the little 
teeth on each side of the centre one are short and robust, those further out are much 
more produced. 
In addition to the above lingual organ, we find superadded in this species a collar 
of crowded minute bifid spines arranged on the anterior surface of the buccal lip; 
to this curious organ we shall again shortly refer. 
In D. hilamellata we have a very interesting modification of the buccal apparatus. 
* Plate XII. fig. 8. t Plate XII. fig. 9. i Plate XII. fig. 10. § Plate XII. figs. 11. 12 and 13. 
