INTRODUCTION. 7 



TERMINOLOGY. 



It is somewhat difficult to obtain suitable terms for the different aspects of the 

 starfish body. The older observers termed the mouth side ventral as it was 

 turned towards the earth. The upwardly directed surface was called dorsal. 

 Unfortunately this terminology, although readily understandable, does not lend 

 itself to the uecessary morphological comparisons with the Pelmatozoa in which 

 the mouth is directed upwards. I have in consequence adopted Bather's suggestion 

 (compare 8, p. GO) that the mouth side should be called "oral" and the opposing 

 side " apical." The names convey a more definite meaning to the lay reader than 

 do the terms " actinal " and " abactinal " used customarily in this connection and are 

 less easily confused. It is also useful to have terms which will give the orientation 

 of the faces of individual ossicles of the ambulacral groove. 



The " proximal " face is that nearest to the oro-apical pole. 



The " distal " face is that farthest from the oro-apical pole. 



The " inner " face is that nearest to a median line drawn the Avhole length of 

 the arm. 



The " outer " face is that farthest from the same line. 



THE GENERAL PRINCIPLES OF CLASSIFICATION OF THE 



ASTEROZOA. 



Because of poor material the great majority of observers have had to rely, in 

 the main, on the general form of the fossil Asterozoa when classifying them. Only 

 the most recent work, such as that of Jaekel, Schondorf, Sollas and Sollas, gives 

 details which are sufficiently full to establish true relationships. The general 

 appearance of the fossils may be very misleading, and it is only careful investiga- 

 tions of the minute structure of the ossicles which are of critical value. 



(a) The Significance of tlie " Asteroid " Body. — Among recent Asterozoa there 

 are two types of body-form : the one " asteroid " (star-shaped), almost without 

 exception characteristic of the Asteroidea, the other " disc-shaped," peculiar to the 

 Ophiuroidea. In early Palaeozoic times these characters had not the same 

 significance. The " asteroid " type of body appears to be the primitive form 

 common to the more lowly organised members of all the early families. The sharp 

 separation between arm and disc arises later as a secondary modification. Embryo- 

 logy suggests this, for in the very young Ophiuroid (MacBride, 43, p. 478) it is 

 found that the arms melt into a small central disc as in the starfish. Paleeontology 

 confirms the supposition. At one time all Palseozoic forms with an asteroid body 

 were classified among the Asteroidea, and in consequence this class appeared to be 

 well represented in Palaeozoic times. Schondorf has shown, however, that certain 

 of these forms, as, e. r/., Aspidosoma (67) and Sturtzaster (68, p. 220), are widely 



