38 



BRITISH PALAEOZOIC ASTEROZOA. 



When one exainiiies these early forms one is struck hy the hirge number of species 

 in which the disc is small compared with the arms. Such a form is Falgeasterinn 

 primsexa (Text-fig. oO). At the first sight the rnadreporite appears to be on the 

 oral surface. If we examine the form more closely we find that the interradial 

 area, in which is placed the madreporite, does not really belong to the oral surface, 

 but has been squeezed by post-mortem compression into that position. The 

 madreporite was really marginal and originally occupied a position just dorsal to 

 the adambulacralia. 



In fact, the madreporite is exactly in the position observed in Bury's figures of 

 newly metamorphosed Asteroid larvte (11, pi. vi, figs. 21, 22, 24). 



The hypothesis advanced here also renders it comparatively easy to follow the 

 dorsal migration of the madreporite in the Asteroidea. The primitive position of 

 tliis ])late must have been at al)out the point 7. ///•. of Text-fig. 20. The plate 



Text-fig. 31. — Diiigraminatic cross - section of disc of an Ophiuroid (after Lang). 1, dorsal wall of disc; 

 2, bnlg-ing- of the digestive sac : 3, bursa ; 4, gonad on the bursal wall ; o, base of the arm ; 6, vonti-al wall 

 of the disc ; 7, genital slit ; 8, genital plate ; il, bursal scale. 



had only to travel a short distance through the slightly calcified interbrachial 

 area to its final destination. It is possible that this dorsal migration was brought 

 about when the tube-feet began to be used both for walking and for pulling. The 

 strong cilia placed in the pores of the madreporite and in the stone-canal would 

 tend to become clogged Avith mud if the current of water which they produce 

 originated too near the sea bottom. The efficiency of the water-vascular system 

 would thus be impaired, as the tube-feet could no longer be kept tensel}^ filled 

 with water. 



An adapical migration of the madi'eporite occurred also in other branches of 

 the primitive stock, e <j. in Tropiihixtcr prctinatns, a Liassic representative of a 

 family the characteristics of which will be descril)ed in later portions of this 

 monograph. 



Genital BnrsR' in. flic Aspidosoiuatida'. — I found on investigating Aspido><omi< 

 graijce that there was good evidence of the presence of genital bursa? in this group. 

 The species is represented by about five or six well-preserved specimens, and the 

 disc of each was found to l)e crowded with young. The only explanation which 

 can be offered is that the form was viviparous, as are certain Recent Ophiuroids, 



