10 BRITISH PALEOZOIC ASTEROZOA. 



A somewhat full account of the structure of each of these skeletal parts in 

 Recent forms is given before certain typical fossils are dealt with. It is felt that 

 only by such careful comparison can the true meaning of the modifications be fully 

 understood. 



A. THE OSSICLES OF THE ARM. 

 The Structure of the Arm in Recent Asteroidea. 



(i) Tlie General Structiwe. — The diagram given (Text-fig. 2) shows the general 

 arrangement of the ossicles as seen in cross-section. The most important ossicles 

 are the ambulacralia and adambulacralia, for these ossicles or their modified equi- 

 valents are found in all Asterozoa. The ambulacralia, which in the Pelmatozoan 

 ancestor (Text-fig. 1) acted merely as flooring plates to the ambulacral groove^ 



J6. pi. 



jZJooao oocSoo oo' 



Am. ii%^''^'^P- 



/Mi. 



'1700BJ Lb^^^ _ 



Ma;- ^^ '^'Ad. i:l 



Text-fig. 2. — A diagrammatic cross-section through the arm of an Asteroid. Am , ambvilacral; Ad., adamhii- 

 lacral ; Am. Ch., ambulacral channel ; 4mp., ampulla ; Ab. pi., ajpieal plates ; S. M., supero-marginalia ; 

 I. M., infero-marginalia.; V. L., ventro-lateralia ; J/,, dorsal transverse muscle; J/;, dorsal longitudinal 

 muscle ; M^, adambulacral longitudinal muscles ; If,, vertical muscle. The ventral transverse muscles are 

 not shown. They lie just ventral to the ambulacral channel. 



now form a high arch calculated to give firm support to the pull which is exerted 

 through the tube-feet. The adambulacralia are no longer merely covering plates, 

 but serve as foundations to the arch, with which they are intimately bound by means 

 of strong vertical muscles. Spines are carried on the adamliulacralia in order to 

 protect the delicate organs in the groove. The groove can be opened for the 

 egress of the tube-feet l^y means of transverse dorsal muscles, which by their 

 contraction force the lower ends of the arch outwards. Closure can be effected 

 by means of transverse ventral muscles. The arm can be bent by means of 

 longitudinal muscles which stretch between the adambulacralia. The necessity 

 for mechanical efficiency in these movements is shown b}^ the following account of 

 the manner in which the Asteroid eats. 



MacBride (43, p. 439) states with respect to Asterias ; " The starfish seizes its 

 j)rey by the tube-feet and places it directly underneath its mouth, folding its arms 

 down over it umbrella-fashion. The muscles which run around the arms and disc 

 in the body-wall contract, and the pressure thus brought to bear on the incompres- 

 sible fluid contained in the coelom forces out the thin membranous peristome and 



