INTRODUCTION. 3 



by Stiirtz (7()-8<>). Opportunity has been taken to compare the structure of 

 these with that of species from British rocks. 



I am greatly indebted to the officers who have kindly made it easy for me to 

 examine the various collections thoroughly. 



THE POSITION OF THE ASTEROZOA AMONG THE 

 ECHINODERMATA. 



The Gtknkual CiiAUACTKrviSTics of the E(jhinodei;.\i.vta. 



The consideration of the essential characters of an Echinoderm will help us to 

 understand the position of the Asterozoa. These characters have been well 

 portrayed both by Bather (3, 4, D) and by MacBride (43). 



(i) There appears to be no doubt that the Echinodermata are descended from 

 bilaterally symmetrical forms which fixed themselves to the sea-floor. This 

 fixation gradually impressed a more or less radial symmetry upon the organs. 



(ii) The fixed forms were originally pear-shaped. The narrow end of the 

 pear was a stalk, the more globular portion contained the gut and liody-cavities. 

 Food was driven to the mouth by means of currents of water created b}^ the 

 movement of cilia. 



(iii) A division of the body-cavity (the hydrocoel) underwent a process of 

 specialisation to produce a water- vascular system. This took the form of a ring- 

 shaped canal which embraced the mouth and gave off radial canals (usually five in 

 number) which ran to the periphery of the body (compare Text-fig. 1). Lateral 

 branches came off from the radial canals and pushed out the skin so as to appear 

 as the appendages of the body known as tube-feet or podia. The water-vascular 

 system communicated with the exterior through a. canal (the pore-canal) which 

 opened outwards through a water-pore. The pore-canal was lined with poAverful 

 cilia and kept the water-vascular system tense with sea-water. 



(iv) The tube-feet could not at first have served the purpose of locomotion. 

 In the modern fixed forms they are covered with cilia, and these cilia help the 

 cilia of the body in creating currents which waft the food to the mouth. We are 

 therefore justified in surmising that the podia at first merely originated as 

 outgroAvths of the body, the purpose of which was to obtain as great a ciliated 

 expanse as possible. 



(v) The soft parts became protected by calcareous plates embedded in the 

 flesh. 



The forms of this type were the most primitive of the Pelmatozoa. 



Evolution now appears to have proceeded along two divergent lines to produce 

 the remaining Pelmatozoa and the Eleutherozoa. 



A. The remninhuj Pelmatozoa. — These retained the primitive method of feeding. 



