INTRODUCTION. 35 



acquii'ud .secoiulurily and iiidopoiulentl}' in iiuuiy forms. Tlu; [)rosence of the 

 apical system is worth noting-, however, becanse the madreporite in trne Asteroidea 

 is fonnd very frequently at the distal end of one of the primai-y interradialia 

 (Text-fio-. 29). 



Tin' liifcrhrachinl Area. — Schondorf has shown that many Palaeozoic Asteroidea 

 have a weakly calcified area in each interradins. The area is l)ounded apically by 

 the supero-marginalia, and orally by the infero-marginalia. Usually in Recent 

 forms these two maro-inal series touch throuo-hout their entire lenofth. The inter- 



•■-J o o 



brachial area may have been of use for respiration. 



D. THE MADREPORITE. 



The study of the madreporite in fossil Asterozoa is most important, as its form 

 and position enable us to decide many questions of relationship. The madreporite 

 is, as is well known, the perforated plate through which water is conveyed, by 

 means of a vessel known as the "stone-canal," to the water-vascular system. The 

 madreporite, in Recent Asteroidea, is r//>/<v^/, large, and has characteristic striations. 

 In Recent Ophiuroidea, on the contrary, there is usually no special striated plate, 

 but the stone-canal opens to the exterior through a small aperture in the ovdllij 

 situated l)uccal shields (see p. 16). 



Many fossil Asterozoa do not appear to possess a madreporite, and it is 

 probable that in those cases the water pore opened to the exterior through small 

 openings unrecognisable in any but exceedingly well-preserved specimens. Other 

 fossils show a madreporite that is neither oral nor apical but marginal in position. 



[a) The tSize of the Madreporite. — MacBride (43, p. 487) has pointed out that 

 the size of the madreporite appears to be governed by the extent to which the 

 tube-feet are used. In the Asteroidea the tube-feet are large and are constantly 

 being vigorousl}^ extended. There is in consequence a loss of fluid by transudation. 

 In the Ophiuroidea the tube-feet are merely sense-organs and the extension is no 

 longer a vigorous act. The loss of water is much less and the madreporite is 

 much reduced. Many Palaeozoic Asterozoa do not appear to have a madreporite. 

 I am inclined to regard this, in these old forms, as usually a primitive feature. On 

 the other hand, a conspicuous madreporite is developed not only in the majority 

 of the true Asteroidea, but in many Palaeozoic Ophiuroid-like forms, as, e. g., 

 Lapivorthiira m.iJtonl, Salter, and in various species of Protaster. This suggests 

 that the tube-feet in these forms were more actively concerned in locomotion than 

 in Recent Ophiuroidea, a feature also suggested by the comparatively large size of 

 the branch canals. 



At a later period the perfection of the " wriggling " movements of the arm 

 rendered these " walking " functions of the tube-feet unnecessary, and led to a 

 secondary reduction in the size of the opening of the stone-canal in the Ophiuroidea. 



