INTRODUCTION. 5 



Asterozoa. To do tliis I carefully collated and compared the observations made 

 by embryologists with my own researches upon the new fossil material. The 

 results suggest that the ancestor had the stalk and general form demanded by 

 MacBride and ambulacral grooves with the fundamental characters described 

 by Bather. 



The attempt is at any rate useful, in so far as it enables the reader readily 

 to visualise a starting-point from which he can follow later developments. 



CP 



MAP 



AmCA. 



Tkxt-fig. 1. — Diagramniatie reconstruction of the Pelmatozoan ancestor of the Asterozoa. Arms I and V are 

 drawn with the covering plates stripped away ; Arm V with the mouth-angle plates also removed. C. P., 

 covering plates ; F. P., flooring plates ; M. A. P., mouth-angle plates ; T. P., terminal plate ; iv. v. r., water- 

 vascular ring ; p.c, pore canal ; r. v., radial water- vascular vessel ; w. p., water pore ; Am. Ch., ambulacral 

 channel ; alt., point of attachment to the sea floor. 



The " Fixed " Ancestok oe the Asterozoa. 



Text-f]g. 1 is a diagram of the ancestral form, shown flexed on a stalk, issuing 

 from the oral surface from a point inside the water- vascular ring. 



(i) The stalk was well marked. MacBride has shown a stalked stage in the 

 development of the recent starfish Asterina (42), and Mead (45, j). 212) has 

 shown the same stage in the common starfish Asterias. 



(ii) The disc is small compared with the arms (see the argument from the 

 ■position of the madreporite, pp. 36, 37). 



(iii) The ossicles associated with the water-vascular system are in two series : 

 {a) A " flooring " series arranged to form the sides of a groove (the ambulacral 

 groove), with the main branch-canal of the water- vascular system at the bottom 

 of the groove and the podia retractile under cover of its walls, [h) A " covering " 

 series serving as an additional means of protection to the soft structures (see p. 10). 



(iv) Primitively the podia were arranged not exactly opposite to each other. 



