MONOGRAPH OF THE EXISTING CRINOIDS. 307 



is patent, with its edges everted and crenulate, and the tube leading to it quite 

 shrunk and flaccid. 



In tlie species of Comasteridse in which the mouth is interradial the main 

 ambulacral trunk which passes to the left from it is considerably longer than 

 that which passes to the right (fig. 685, p. 341), as a natural result of the displace- 

 ment of the mouth. A line passing through the mouth and the anal tube passes 

 also through the center of the left posterior ray (part 1, fig. 26, p. 69), though 

 this ray is supplied entirely from the left ambulacral trunk. If this line be taken 

 as representing the antero-posterior axis the left groove trunk is seen to pass to 

 all the arms of the left side, and thence onward to all the arms of the posterior 

 ray, half of which lie on the right side, while the right groove trunk passes only 

 a part of the way down the right side. The former thus supplies three rays 

 and the latter two. But almost invariably when this occurs the ambulacra of 

 the posterior arm are suppressed (part 1, fig. 27, p. 69), so that in reality each 

 of the primary trunks supplies two rays. When the mouth is radial it is in line 

 with the anal tube and the center of the posterior interradius (part 1, fig. 25, 

 p. 69), and in this case each of the groove trunks supplies two rays, the anterior 

 ray being furnished with grooves which lead directly to it from the mouth as 

 in Antedon. 



Among the species of Comasteridse in which the mouth is radial and the two 

 lateral groove trunks therefore of equal size, as in Gomatula rotalaria (fig. 688, 

 p. 341), the metamorphosis of the two posterior rays, by which the ambulacra, 

 epithelial nerves, and associated structures are lost, takes place at an equal rate; 

 but when the mouth is interradial, as is usually the case in multibrachiate species, 

 the left posterior ray, which is farthest from the mouth, is always affected first 

 and to the greatest extent (figs. 700, 699, p. 341). Very many cases are met with 

 in which this is the only ray to be affected, so that a peculiarly complicated sym- 

 metry has resulted, in which the right anterior interradius (including the mouth) 

 is anterior, and the reduced and modified left posterior ray is posterior, the plane 

 of the bilateral symmetry superposed upon the fundamental pentamerous sym- 

 metry thus having been shifted 36° from the normal in which radius A is anterior 

 and the opposite interradius posterior (part 1, figs. 25-28, p. 69). With the atrophy 

 and metamorphosis of the left posterior ray and the disappearance of its ambu- 

 lacral structures this becomes an isolated ray with similar interradial areas on 

 either side, each of which is bounded on the further side by a ray supplied with 

 ambulacral grooves derived from a trunk coming from the mouth which is of 

 the same length as the trunk on the opposite side of the disk. Since the anal tube 

 in either case is approximately central, so far as the structures of the disk and 

 rays are concerned, this symmetry, when developed, is just as perfect a bilateral 

 symmetry as the normal in which ray A lies in the antero-posterior axis. 



The relation of the ambulacral grooves to the brachials varies greatly in the 

 different genera and species. Carpenter noted that the middle line of the upper 

 surface of each segment of the brachial skeleton is occupied by a groove of variable 

 width and depth which is bordered on each side by the more or less prominent 



