94 INVERTEBRATE PALAEONTOLOGY 



necessary if morphological research is to be co-ordinated 

 and made intelligible; tentative schemes of phylogeny 

 serve as convenient, if imperfect, correlations of estab- 

 lished data in morphogeny. 



On PI. vii. figs. 2-13 an attempt is made to indicate 

 the nature of a morphogenetic sequence, and to show 

 its relation to phylogeny. The apical systems of 

 Irregular Echinoids undergo disorganization (in onto- 

 geny and morphogeny) by retrogression of the periproct. 

 The posterior genital plate is resorbed ; the gap in 

 the cycle, and the central area once occupied by the 

 periproct, have to be filled. Figs. 2 and 3 show two 

 Diademoid (Regular) types of apical system ; in both 

 the backward tendency of periproct-movement is ap- 

 preciable, but does not disturb the genital and ocular 

 rings. (The alternative arrows connecting these figures in- 

 dicate uncertainty as to the morphogenetic relation of 

 Diademine and Calycine systems.) In Plesiechinus (fig. 

 4), the morphogenetic (and apparently phylogenetic) 

 ancestor of Irregular Echinoids, the periproct occupies 

 the posterior part of the system, breaching the cycle 

 and displacing the surviving plates. From this con- 

 dition two main trends of recovery can be traced. In 

 that shown by the sequence of figures 9-11, no attempt 

 is made to restore the lost genital ; its place, and the 

 centre of the system, are filled by extension of the 

 madreporic plate. A slight modification of this method 

 (figs. 12 and 13) closes up the system after the periproct 

 has passed to the rear, producing in its extreme phase 

 an elongate apical system with alternating plates. The 

 second trend of recovery is shown in figs. 5-8. Here 

 a new plate is formed in the wake of the receding 

 periproct ; in course of time, it assumes the proportions 

 and function of the missing genital. The centre of 

 the system is filled by inward extension of the genitals, 



