EOACTINID^]. 183 



to the ambulacral channel, and afforded points of attachment for the ventral cross- 

 muscle which closed the groove. We can see that this muscle must have been 

 much less powerful than in the recent Asteroidea if we compare the slight con- 

 cavity shown in the text-figure with the deep pit (V.T. unite.) of Text-figure 125. 

 The place for the position of the passage of the branch from the radial water- 

 vascular vessel to the tube-foot is indicated by the cut-away portion of the posterior 

 branch of the longitudinal ridge. This position differs from that in recent 

 Asteroidea (Text-fig. 125), where the branch is exactly between the ambulacralia 

 — a condition which Ludwig (39, p. 355) regarded as primitive. The point will be 

 returned to later. 



.M. 



Ad. 



Am.CK. 



Text-fig. 124. — Floor and inner wall of the ambulacral groove of Schuchertia wenlocJci. Ad., adambulacral ; 

 Am. Ch., ambulacral channel ; B. Tf., branch of radial water-vascular vessel to tube-foot ; D., distal 

 direction ; I.M., infero-marginal ; P., proximal direction. This figure should be compared with Text- 

 fig. 119. x 18. 



Bather (90, pp. 317, 318) has shown that there is considerable resemblance 

 between ambulacralia of this type and the flooring-plates of the groove of Ed r to- 

 aster. His comparison is as follows : " In the older Asterozoa, from which the 

 true Asteroidea and Ophiuroidea were derived, the plates which in a modern star- 

 fish are known as ' ambulacralia ' were ' little more than mere flooring-plates to the 

 ambulacral groove' (Spencer, p. 21). They formed a double series, either opposed 

 as in recent Asteroidea, or alternating as in Edrioaster. Spencer, following 

 Gregory and Jaekel, regards the latter arrangement as the more primitive. These 

 ambulacrals were approximately rectangular in plan, and excavated along the per- 

 radial sutures by a shallow ' ambulacral channel ' for the radial water-vessel. 

 Along the sutures at right-angles to this the plates were deeply excavate, leaving a 

 well-marked median transverse ridge along each. The longitudinal ridge, parallel 

 to the ambulacral channel, was but slight, indicating the feeble development of the 



