Parts of the Lantern of Aristotle in Echinus miliaris. 481 



leading factor in the evolution of a high lantern is perhaps seen in the lantern 

 of the metamorphosing pluteus (Plate 11, fig. 1„ L ). Here we find that, even 

 while the two jaw-rudiments are in the tri-radiate spicule stage, the tooth is 

 in a relatively higher state of development with the first cone and nine 

 pairs of secondary lamellae ; and it extends aborally beyond the limits of the 

 jaw-rudiments. It is perhaps admissible to infer from this 'that, with the 

 growing length of the tooth, the jaw-rudiments kept pace pari passu, and 

 the result was a lantern of great height. 



" Summarising the foregoing conclusions, I regard the lantern of Aristotle 

 as homoplastic with the buccal armature of star-fishes ; the pyramids are the 

 modified first adambulacral plates ; the epiphyses have arisen from the first 

 ambulacral plates of the Echinoid series ; and the teeth represent the 

 odontophore, which has acquired a persistent root ; the radius and rotula 

 remain problematical." So wrote Prof. Sollas in the year 1899 in his paper 

 on " Silurian Echinoidea and Ophiuroidea " (5). I leave the reader to 

 compare this statement with what has been indicated on the homologies of 

 the lantern of Aristotle in this paper. It will be seen that ihe idea of the 

 homology of the urchin-tooth advanced by me is entirely new, and based on 

 indisputable embryological facts. 



9. A Comparison between Gicsbrceht's (1)* Account of the Development of the 

 Urchin-tooth as revealed by its Boot-end in the Adult and the Construction 

 of the Same as seen in the Young Urchin. 



Attempts to probe into the peculiar construction of an urchin-tooth were 

 made even as early as the year 1841. Beginning with Valentin.f Meyer, 

 Waldeyer, Leuckart, Giesbrecht, Loven, Theel and MacBride successively 

 each gave his attention to this organ. With the exception of Loven, Theel 

 and MacBride, who studied it in young urchins, these authors employed in 

 their researches the root-end of the adult urchin-tooth. The credit of our 

 knowledge that the urchin-tooth is built up of lamelhe is largely due to 

 these workers. Nevertheless, our ideas as to the manner of the origin of 

 these lamellse, their growth, arrangement and significance have hitherto been 

 rather imperfect. The first attempt, I may say, to interpret the tooth- 

 structure in terms of biogenetic laws has been made in this paper (see 

 Discussion). 



A very interesting study was made by comparing my own observations, 

 based on the embryonic tooth, with those obtained by the examination of the 



* The greater part of this important paper was translated to me by Prof. MacBride. 

 t "Anatomie du genre Echinus," 1841, by G. Valentin, the first monograph in 

 ' Monogr. d'Echinodermes viv. et foss.,' published by L. Agassiz. 



VOL. XCIII. — B. 2 M 



