1904] Affinities of Palwodiscus and Agelacrinus. 45 



The justice of these observations is strikingly illustrated by the 

 discovery of the double series of plates in Paleeodiscus. The lower 

 series of ambulacral plates in Palseodiscus must be regarded, just as 

 the auricles of Echinus and the ambulacral processes of Cidaris, as the 

 representatives of the ambulacral plates of the Asteroids. 



Loven (11, p. 38) has shown " that the auricles of the Ectobranchiates 

 are seen to adhere to the ambulacra as distinct and independent parts 

 joined mainly to them by means of articulation, and to increase by their 

 own growth, both surfaces, the auricular and the ambulacral, during 

 the constant fluctuations of absorption and renewal mutually main- 

 taining their firm connection." 



This statement shows that embryology justified the homologies drawn 

 above, and that we have in the embryo two distinct series of plates, 

 the outer of which are the true Echinoid plates, the inner which become 

 auricles are the Asteroid ambulacrals. 



Semon and Lang have suggested that the true Echinoid plates may 

 be derived by the overgrowth of the adambulacral plates of Asteroids,, 

 a view which would support this theory. 



We must conclude, therefore, that whilst a double series of plates, 

 namely, an outer series peculiar to the Echinoid, perhaps derived from 

 the adambulacrals of Asteroids, and an inner series homologous with 

 the plates of an Asteroid occur in the ambulacrum of all the Echinoids, 

 a complete double series occurs in Palseodiscus, which is therefore 

 entitled to be placed at the base of the Echinoid stem. It is not alone 

 in the structure of the ambulacra that Palaeodiscus shows itself to be 

 the most primitive of Echinoids, for we have seen that the inter- 

 ambulacral areas present many ideally primitive Echinoid structures. 



That the transition must have been from Asteroid to Echinoid 

 and not vice versd, the possession of a lantern of Aristotle by the latter 

 forms is quite sufficient proof. 



MacBride (12, p. 316) in a recent paper has adduced embryological 

 evidence to derive the Echinoids from the Asteroids ; he shows : " That 

 in the just metamorphosed sea urchin the humped dorsal surface is- 

 greater in extent than the flat ventral one, and the radial canals extend 

 straight outwards from the water vascular ring and end in prominent 

 terminal tentacles, both of which are Asteroid features." MacBride also 

 remarks upon the "highly developed Pedicellarise amongst the 

 Echinoidea." Amongst Asteroidea they are by no means universal, and 

 numerous transitional forms are found showing their origin from the 

 spines; in a word, pedicellarise have been developed amongst the. 

 Asteroidea and inherited ready-made, so to speak, by the Echinoidea. 



As to the Pelmatozoan ancestor of the Eleutherozoa it is better tc 

 leave that in the realms of controversy, Asteroids are known from the 

 Cambrian period, and it is very probable that this ancestor was. 

 pre-Cambrian. 



