706 PKOr. W. J. SOLLAS ON SILIJKIAN [NoV. 1 899, 



Asteroidea, and Echinoidea, we find a marked distinction between the 

 first and the two latter, in the fact that the latter possess adhesive 

 tube-feet, bj^ which they are able to hold fast to the ground, and 

 thus move surely though very slowly. The power of a large 

 Echinoid to hold on to the perpendicular face of a rock, even when 

 exposed to a heavy ground-swell, must have been often observed 

 with surprise by the shore-collector. The Ophiuroid, on the other 

 hand, is without adherent suckers, but makes np for their absence 

 by its power of rapid movement. The structure of the brittle-star 

 is in complete adaptation to this faculty ; and its ambulacral ossicles, 

 originally similar to those of a star-fish, have become modified to 

 serve as the well-known vertebral column, which affords both 

 protection to the nerves and vessels of the arms and attachment 

 to the powerful longitudinal muscles by which these organs of 

 locomotion are flexed. 



The Echinoids and Asteroids are equally well adapted to their 

 respective modes of progression. The tube-feet are lines along which 

 tension is active ; the pull is resisted at one end by the ground, at the 

 other by the skeleton, which is constructed in an ideally perfect 

 manner to withstand the pull. In ancient star-fishes the ambulacral 

 ossicles were mere trabeculae lying parallel with the flat ventral 

 surface, a very inefficient position : this, in the evolution of the race, 

 they have exchanged for that which they now possess, namely, 

 arrangement in the form of an arch. This arch is so deep that it not 

 merely furnishes the required amount of resistance, but securely 

 protects the underlying nerve and water-vessel. In the Echinoids 

 such an arch would be rather a source of weakness, as interrupting 

 the continuity of the closed, more or less spherical form, which is 

 of itself sufficient to supply the requisite amount of resistance. 

 Yet, if the ambulacra of Echinoids and Asteroids are to be considered 

 homologous, it may be inferred that such an arrangement was at 

 one time in existence. In such a case, what would be the probable 

 course of transition from the primitive state to that which at 

 present obtains ? One might conjecture that the discontinuity in the 

 spherical form would be bridged over by a tangential growth, either 

 of new plates, or of processes from the bases of those already 

 existing. After this new growth had arisen, the original ambulacral 

 plates, included within the test, would become functionless and 

 might be dispensed with ; as useless organs they might be expected 

 to disappear. Thus, by hypothesis, ancient urchins should have 

 existed in which the ambulacral skeleton would unite both Asteroid 

 and Echinoid characters. It was this consideration that led me to 

 investigate closely the ambulacral region in Palceodiscus, with, as 

 we have seen, confirmatory results. 



The structure of the ambulacra in Palceodiscus forcibly recalls 

 that of Mesites,^ which has likewise been regarded as a link between 



^ Mesites Pusireffskii, Hoffmann, Verh. Mineralog. Gesellsch. Petersb. ser. 2, 

 Tol. i (1866) p. 1 & pi. i ; F. Schmidt, Mem. Acad. Imp. Sci. St. Petersb. ser. 7, 

 Tol. xxi (1873) No. 11, p. 34 & pi. iii ; Neuraayr, * Die Stamme des Thierreiches,' 

 1889, p. 420 ; Steinmann & Doederlein, ' Elemente der Palaontologie,' 1890, 

 p. 181 ; Haeckel, ' Festschrift z. 70ten Geburtstage v. Carl Gegenbaur/ vol. i 

 (1896) p. 119. 



