INTRODUCTION. 9 



ment or mode of life. It appears to me that these principles also hold true for the 

 older forms. 



This separation into the various lineages must necessarily involve considerable 

 descriptive detail which would be out of place here. For the moment a satisfactory- 

 provisional classification, which gives a broad view of the general lines of 

 J evolution, can be obtained from a consideration of the mode of life of the forms as 



judged by the structure and arrangement of the ossicles of the ambulacral groove 

 and the mouth-frame. This classification has the further advantaofe that it is 

 based upon characters which are familiar to us from a study of Recent forms. 



(c) Tlis Structure and Arrangement of the Ossicles of the Ambulacral Groove and 

 tire Mouth-frame. — The observations of MacBride (43) assist greatly here, and 

 following his arguments we can divide Recent Asterozoa into tAvo groups : 



(1) llie " Graspers.^^ — Asteroid forms in which the tube-feet are used for 

 walking, and for grasping and pulling open the bivalve shells of the molluscs upon 

 which they usually feed. The ambulacralia form an arch to take the pull. 



(2) The " Wrigglers.'" — Ophiuroid forms in which the tube-feet have lost 

 locomotory powers and iDecome much reduced. The animals progress by wriggling 

 movements of the arm, and the ossicles of the ambulacral groove are extensively 

 modified for this purpose. The food is pushed into the mouth by the first two 

 pairs of tube-feet, which become considerably enlarged and. are known as buccal 

 tentacles. 



If Ave trace the history of the forms backwards Ave find that the difference 

 between them tends to disappear. Both the "graspers" and the "wrigglers" 

 descended from a third group Avhicli I call provisionally " the primitive Asterozoa." 

 Some of the least specialised forms of this group used their tube-feet not for 

 grasping but for passing small particles of food to the mouth, as did their 

 Pelmatozoan ancestor. 



The Palseozoic " wrigglers " are not a homogeneous group, but contain at least 

 two series of forms wdiicli have undergone a most remarkable analogous 

 (homoplastic) course of development. One of these series had alternating 

 ambulacralia (Aspidosomatidge), the other opposite ambulacralia (Lapworthuridge). 

 For the present both the series are included in the Ophiuroidea. 



THE GENERAL PLAN OF DESCRIPTION. 



It is couA^enient for the purposes of description to subdivide the skeletal parts 

 of the Asterozoa as follows : 



A. The ossicles of the arm. 



B. The mouth parts. 



c. The accessory ossicles of the disc. 

 D. The madreporite. 



2 



