304 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



The primary groove trunks of Eudiocrinus (fig. 712, p. 346), Pentametocrinwi 

 (figs. 758, p. 353, and 1158, pi. 25), and Thaunuitocrinus (fig. 759, p. 353) continue 

 out upon the arms undivided. On the disk those of the first two are equidistant, 

 but those of the last, though entirely separated from each other, are arranged in 

 five pairs, indicating the five primary radial divisions, which have become sub- 

 divided to form the 10 rays. 



The five interpalmer or interambulacral ai-eas delimited by the ambulacral 

 grooves in their passage across the disk are usually nearly of the same size, though 

 that in which the anal tube is situated is commonly slightly larger than the others, 

 and the ambulacral grooves bounding it are slightly bowed outward. In large 

 species, however, in which the mouth is more or less excentric, as in the large 

 species of Heliometrinae and in Tropiometra, the anal area is sometimes consider- 

 ably larger than any of the others and is bounded by noticeably convex ambu- 

 lacral grooves. 



From the disk the ambulacral grooves extend outward along the ventral sur- 

 face of the arms, and from these brachial extensions lateral branches are given off, 

 which reach nearly to the tip of each of the pinnules, with the exception of those 

 borne by the division series (when present) and the lowest brachials. The floor 

 of the grooves is composed of ciliated epithelium with a subepithelial nerve band, 

 beneath which lie the radial blood and water vessels. 



The sides of the grooves are not straight, but are scalloped, showing con- 

 tinuous series of minute valvules or lappets, the crescentic or respiratory leaves of 

 Sir Wyville Thomson. At the base of each lappet, and to some extent protected 

 by it, is a group of three tentacles, one of which, the most distal, is larger than 

 the other two. This trifid group of tentacles and the cavity of the respiratory 

 leaf adjacent to them receive a common branch from the radial water vessel. These 

 groups of tentacles alternate on the opposite sides of the ambulacral grooves from 

 the base to the tip of each of the arms and pinnules, and are distributed in the same 

 manner along the sides of the ambulacra of the disk, though here they are not so 

 well developed, especially near the peristome, where every lateral branch of the 

 water vessel supplies only one tentacle. The lappets at the sides of the groove are 

 also far less distinct than on the arms, the edges of the folds of perisome bounding 

 the grooves being only faintly marked by an indistinct wavy line. 



P. H. Carpenter noticed that the conditions described above in many of the 

 Comasteridse applj' only to the arms of the two anterior radii, A and B, and to 

 more or fewer of the antero-lateral arms, C, and E^. The arms of the posterior 

 radius, D, and of the posterior divisions of the lateral radii, Cj and Ej, are often 

 entirely devoid of tentacles, and in many of them the ventral perisome not only 

 exhibits no ambulacral groove (fig. 1037, pi. 12), but is, on the contrary, convex, as 

 in the oral pinnules of Antedon. 



In Comanthus 'parvicirra, as in all comasterids with an interradial mouth 

 (figs. 685, 689, 691, p. 341 and 1149-1153, pi. 24), the anal area is bounded by two 

 large aboral groove trunks which start from the posterior angles of the peristome 

 and form a horseshoe-shaped curve the limbs of which are unequal in size. The 

 smaller right limb is formed by the right lateral ambulacrum, C, while the larger 



