150 HAWAIIAN AND OTHER PACIFIC ECHINI. 



pyga and Micropyga are intermediate in this respect. The lantern of 

 Astropyga is surprisingly like that of Asthenosoma Jjimai. There is great 

 individual diversity in both families in the amount of calcification in the 

 perignathic girdle. In some specimens the auricles are very moderate and 

 the apophyses almost wanting, while in others the auricles are enormous 

 and the apophyses stout. 



In the internal anatomy the two families are much alike, except that 

 the Echinothuridoe have well-developed Stewart's organs and longitudinal 

 body-wall muscles, both of which are rudimentary or wanting in the Dia- 

 dematidte. The Stewart's organs are undoubtedly a heritage, and their 

 form would seem to indicate that they now have little functional impor- 

 tance. The body-wall muscles have doubtless been developed in connec- 

 tion' with the increasing mobility of the test. The gills of the two fami- 

 lies and the sphajridia show the greatest similarity. 



The spines are fundamentally alike in their structure, commonly hol- 

 low, though not infrequently more or less solid, at least near tip. They 

 are much longer in many Diadematidaa than in the Echinothuridaa, but 

 Astropyga and Micropyga are more like the latter family. The presence 

 of poison-tipped spines is a character found in both families, but hoofs 

 and skin-bags seem to belong to the Echinothuridoa exclusively. It should 

 be noted, however, that Dtiderlein has figured spines in Dermatodiadema, 

 which are so formed as to almost warrant their being called " hoofed," 

 and somewhat similar spines occur in Micropyga. Some of the spines of 

 Echinothrix, moreover, might almost be called " sheathed." Such special 

 modifications of the spines cannot be regarded as of great weight in esti- 

 mating relationships. 



The similarities between the two families in their pedicellariaa have been 

 referred to above ; they have also been discussed quite fully by Dbderlein. 

 The fundamental resemblance of the spicules in the tube-feet is noteworthy, 

 though they are much more fully developed in most Echinothuridce than 

 in the Diadematids. In color and size the similarity between the two fam- 

 ilies is more noticeable than the difference. The banding of the primaries, 

 so marked in young Diademas, is noticeable in some Asthenosomas. The 

 Diadematidse and the shallow-water Ecliinothurids are tropical and partic- 

 ularly East Indian forms. The deep-water Ecliinothurids have spread both 

 nqrth and south of the tropics. 



In view of all these facts, we find ourselves driven to the opinion that 



