152 



BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



fourth and succeeding brachials, but a variable number of these earlier pinnules, 

 sometimes only three or four, sometimes as many as forty, resemble in this respect 

 the oral pinnules (part 1, fig. 45, a, p. 79), their ventral surface being convex and 

 devoid of any ciliated epitheliimi or subepithelial band, while their water vessel is 

 simple without any lateral extensions to respiratory leaves and tentacles. In these 

 oral arms, however, branches of the ambulacral groove enter the pinnules sooner 



or later so that the terminal pinnules are 

 always provided with a distinct tentacular 

 apparatus, while the floor of their median 

 groove is of the usual character, consisting 

 of a ciliated epithelium and a subepithelial 

 fibrillar band. 



Carpenter also noticed that in those co- 

 masterids which possess ungrooved posterior 

 arms the gonads upon the pinnules borne 

 by them are usually far more developed than 

 those upon the anterior arms; not only are 

 there more fertile pinnules, though the total 

 number of pinnules may not be more than 

 half that of an anterior arm, but the por- 

 tions of the glands within these pinnules 

 also attain a greater size than in the anterior 

 arms, the basal and median pinnules of the 

 latter being usually less swollen than the 

 corresponding pinnules of a posterior arm. 

 Carpenter found that this ungrooved 

 condition of the lower pinnules may also oc- 

 cur on all of the arms of some of the endo- 

 cyclic forms and that it is especially re- 

 markable in types like Pcecilometra acoda 

 (fig. 1058, pi. 14) and Perissometra angusti- 

 calyx (fig. 1055, pi. 14), which have a 

 strongly plated ventral perisome. The am- 

 bulacral grooves of all the arms and of the 

 later pinnules are well protected by plates, 

 but they do not extend onto (about) the first 

 20 pinnules which bear the large gonads, though the latter are protected by a very 

 close and regular pavement of anambulacral plates. In other species, however, 

 which have equally well-plated pinnules, such as Aglaometra incerta (fig. 1062, 

 pi. 14), the ambulacra extend over their ventral surface in the usual way. 



While in almost all cases the genital pinnules are shorter than the more 

 slender distal pinnules, this is not always true. In the family Tropiometridffi 

 (fig. 279, p. 213) the genital pinnules are rather stout and very long, longer than 

 the distal pinnules, which are unusually short. In the Ptilometrinse (figs. 280, 



-Lateral view of specimen of Steno- . 

 metba diadema. 



