108 HAWAIIAN AND OTHER PACIFIC ECHINI. 



lengtli exceeding 2| times the external circumference of the test. The 

 oesophagus is remarkably long, first bending backwards above the lantern 

 and then turning to the left and running forwards to the anterior ambula- 

 crum, where it enlarges abruptly to form the stomach-intestine. The first 

 or lower coil of the intestine, which runs from left to right (i e., contrary 

 to the hands of a clock), is arranged in a double fold in each ambulacrum 

 except in the anterior one, where only the right-hand fold occurs, the other 

 being replaced by the abrupt, upward and backward turn of the canal. The 

 second or upper coil, running back from right to left above the other, is 

 not so extensively folded, but is yet greatly lengthened by a very deep loop 

 in each interambulacrum. The loop in the left anterior interambulacrum is 

 the smallest, and passes by gradual change into the rectum, which is straight 

 and not noticeably enlarged. The two intestinal coils fit into each other 

 in such a manner that the loops of the upper reach down into the actinal 

 half of the test and lie snugly between the double folds of the lower. The 

 two are united so extensively with each other and with the test by mesen- 

 teries and strands of connective tissue that they are held firmly in place, 

 and can only be separated, without injury, by very careful dissection. 



The Genera and Species of Recent Diadematid^, 



There is still much room for difference of opinion as to the limits of this 

 family when the fossil forms are taken into account, but so far as recent 

 species are concerned, the only point which causes discussion is whether 

 Micropyga should be regarded as one of the family or not. ^lortensen 

 (1904) established a separate family for Micropyga, and Dciderlein (1906) 

 has followed him in this arrangement. We have given very careful atten- 

 tion to the matter, and have nuide extensive comparisons of the internal 

 anatomy of Micropyga, as well as its external features, with other genera of 

 Diadematida3, and we find no sufficient reason for removing the genus from 

 tliat family. The characters upon wliich the fiimily Micropygida? are based, 

 are stated by Mortensen (1904, p. 45) as follows: "This family is charac- 

 terized above all by its anchor-shaped spicules, further by wanting 

 ophicephalous pedicellari:\>, either in the fortn of true ophicophalous 

 or of claviform ones; the triphyllous pcdicollaria? are finely serrate in 

 the outer edge, and the stalk of the pedicellarice consists of several slender 

 rods, almost not united, except at the ends. The tubercles are perforate, 



