ISO BRITISH PALAEOZOIC ASTEROZOA. 



relatively slow and deliberate, while in the Ophiuroidea they are rapid and jerky. 

 Distinct methods of moving the " bones " are used in each case. Ventral flexion, 

 in the Asteroidea, is brought about by simultaneous contraction on both sides of 

 the arm of the longitudinal muscles between the adambulacralia. Muscles which 

 act counter to these are (a) a band running along the dorsal wall of the ccelom to 

 the point of the arm (MacBride, 43, p. 435) ; (l>) dorsal longitudinal muscles 

 attached to the apical faces of the ambulacralia (Text-fig. 128, p. 188). Lateral 

 flexion is brought about by the simultaneous contraction, on one side of the arm 

 only, of the adambulacral and " b " muscles mentioned above. In the recent 

 Ophiuroidea the adambulacralia (side plates) have no muscles between them, and 

 there is no ccelomic band. The whole of the movements are brought about by 

 the "b" muscles andj a new set of muscles (the longitudinal ventral muscles, 



A 



TTl. 



Ap. PL 



Tbxt-fig. 12G. — Wash drawing of a portion of the apical surface of Schuchertia laxata (?), showing exposed 

 ambulacralia. Am., ambulacral ; Ap. PI., apical plates, x 20. 



Text-fig. 13, p. 10). The arrangement allows of independent movement of each 

 "vertebral" sector, and as every sector has its own ganglionic swelling (MacBride, 

 43, p. 488) quick active movement is to be expected. 



There are two points of interest in the evolutionary moulding of the vertebra? 

 — (1) the development of the efficiency of the muscular action ; (2) the parallel 

 changes due to the fact that both the Asteroid ambulacral and the Ophiuroid 

 vertebra have one set of muscles in common, namely, the " b " muscles. The 

 fossil material allows us to follow these changes. 



In the most primitive forms known, e. g. 8. la. rata (Text-fig. 120) or 

 Hudsonaster (Protopalseaster) narrawayi (Text-fig. li>, p. 21), the ambulacralia are 

 closely touching bars of calcite — that is, mere flooring-plates. The longitudinal 

 adambulacral muscles were present, but the only muscle which could have been 

 counter to this must have been that running in the body-wall, of which we can 

 have now no trace. 



8. wenlocki (Text-fig. 127) shows the beginnings of the development of the 



