MONOGKAPH OF THE EXISTING CRINOIDS. 305 



left limb represents the posterior ambulacrum, D, combined witli part or the whole 

 of the left lateral ambulacrum, E. In neither of these limtjs arc the tentacular 

 groups and crescentic leaves so well developed as they are in the two antei-ior 

 ambulacra. After the branches to the two antero-lateral primary arms, Ci and E, 

 have been given off, or sometimes even sooner, the tentacles at the sides of the two 

 aboral groove trunks become more and more insignificant and finally disappear 

 altogether, while the position of the crescentic leaves is only indicated by a very 

 faint wavy line at the edge of each groove. In small specimens with but few arms 

 the grooves of the posterior and postero-lateral arms, D, Cj, and E, may remain 

 in this condition ; but in larger specimens with many arms all traces of the lappets 

 disappear, and the two edges of the groove meet and unite so as to produce a 

 condition in which the ventral surface of the arms and pinnules is convex and does 

 not show the least trace of a groove of any description. The position of the point 

 at which the two folds of perisome bounding the sides of the original ambulacral 

 groove meet and unite varies extremely. The fusion may, though rarely, take 

 place on the disk (figs. 692, 693, 699, 700, p. 341, and 1150, 1151, 1153, pi. 24) ; 

 sometimes it is at the base of the arms, and sometimes not till near their middle 

 or terminal portions. In any case, however, the fusion whenever it occurs is so 

 complete that all trace of the original ambulacral groove is entirely obliterated. 



The position of the mouth in the Comasteridse is subject to great variation. 

 Exceptionally it is central or nearly so; this is invariably the case in Comatilia 

 (fig. 687, p. 341), Comatonia, and Microcomatula, and occurs frequently in Leptone- 

 master (fig. 877, p. 435), Comaater (especially in the smaller species) (figs. 697, 

 698, p. 341), and Comissia^ and not uncommonly in Capillaster multiradiata (fig. 

 1147, pi. 23; compare with fig. 1148). Usually, however, it is marginal or sub- 

 marginal, this position being especially emphasized in the Comactiniince (figs. 

 688-693, p. 341, and 1149, pi. 24) and in Comanthus (figs. 700, 701, p. 341, 1152, 

 pi. 24. and 1154, pi. 25). 



When the mouth is marginal or submarginal it varies from a strictly radial 

 position immediately opposite the center of ray A (figs. 690, 692, p. 341) to an 

 interradial position in the middle of the interradius A — B (figs. 689, 691, 699, 

 701, p. 341) ; generally speaking, the mouth in 10-armed species is radial in position, 

 while in multibrachiate forms it is more commonly interradial, though in each 

 species specimens may occasionally be found in which it is quite radial (figs. 691, 

 692, p. 341). 



The explanation of the interradial position of the mouth may be the following: 

 The ambulacral grooves of necessity divide many times in the multibrachiate types 

 upon the disk as the latter reaches above the last arm division. Economy calls 

 for a maximum concentration of the food particles at the lips, and the fewer the 

 grooves reaching the mouth the greater the concentration will be. It is essential, 

 therefore, that the mouth shall be the focus of the minimum number of food grooves 

 each of which will carry food in the condition of maximum concentration, and 

 each of which will carry approximately the same amount of food. The many 

 coils of the digestive tube force the anterior portion of the organ, and with it 

 the mouth, to a marginal position so close to the edge of the disk that, were it to 



