INTRODUCTION. 29 



Ludw'ig's account of the fate of the ossicles found in the earhest stages of the 

 developing- Ophiuroid is as follows : 



(i) a.2 and ^(/^ fuse to form the jaws. 



(ii) ad^ forms the lateral buccal shield. 



(iii) (ly migrates upwards and forms one of the plates known as the peristomial 

 plates Avhicli fit on the upper surface of the jaws. 



Practically everyone admits the conclusions (i) and (ii), but there is much 

 controversy with respect to (iii). Dr. zur Strassen has pointed out that in very 

 many species the number of peristomial plates is not two as required by the hypo- 

 thesis of Ludwig — a fact which in itself almost disproves the theory. Generally it 

 may be said that from our knowledge of Recent forms the plate known to Ludwig 

 as «! is not definitely recognisable in the adult of either Asteroidea or Ophiuroidea. 



(b) Those suggested hi/ I'alseontology. — Very few observers have studied the 

 mouth-parts of fossil Asterozoa. Jaekel, Schondorf, and Prof, and Miss Sollas 

 have been almost alone in this branch of inquiry. Schondorf has shown that there 

 is no recognisable ossicle corresponding to the a^ of Ludwig in the Devonian 

 Asteroidea. Prof, and Miss Sollas have shown the same fact with respect to English 

 Silurian " Ophiuroidea." These latter observers, however, accept, after a re- 

 investigation of the embryological evidence, the views of Ludwig that there is in 

 the early stages an a^, but they conclude that this ossicle later fuses with a.^. My 

 own search through a large number of forms does not reveal evidence which I can 

 accept as conclusive of the existence of Ludwig's a-^ as a separate ossicle. In the 

 following descriptions I have therefore followed the practice of Schondorf and 

 called the first recognisable anibulacral, a^ This leaves the interradial mouth-parts 

 without a corresponding ambulacral, and they are therefore here always called 

 mouth-angle plates. 



Perhaps later evidence will help finally to decide the matter. At any rate it is 

 possible to confirm the evidence brought forward by Prof, and Miss Sollas that 

 peristomial plates are wanting in Palaeozoic " Ophiuroidea," and that these plates 

 in Recent forms cannot therefore be representatives of embryonic anibulacralia. 



In practically all Ordovician and Silurian " Ophiuroidea " the jaws are Asteroid, 

 inasmuch as the mouth-angle plates have not fused with the first recognisable 

 anibulacralia (compare Sollas and Sollas, 71, p. 22G). They thus retain a condition 

 met with only in the young of modern forms. 



The following account gives some of the lines of change. Generally we may 

 say that there are few changes to be observed in the Asteroidea, but in the 

 Palseozoic " Ophiuroidea " there is {a) the same modification of the mouth-parts 

 for feeding as in Recent forms, while {b) features characteristic of the period 

 are shown inasmuch as (i) the proximal anibulacralia tend to fuse with each other, 

 and (ii) buccal shields have not as yet developed. Various other modifications, 

 however, took place which will be dealt with in a later volume. 



