478 BULLETIN 82, UNITED STATES NATIONAL, MUSEUM. 



The individual plates are not quite parallel to the major axis, for tlie oral 

 ends of the orals and the aboral ends of the basals mark the sides of smaller 

 pentagons than those outlined by the approximated edges of the same plates. 

 These plates therefore appear arranged in a five-sided double pyramid truncated 

 at both apices. 



The relation of the calcareous jjieces to the internal organs has also undergone 

 a change. 



Wliile the basals entirely surround the aboral ccelome the orals have migrated 

 out of the compass of the oral ccelome, so that this now lies between the two 

 systems. 



The pore lies between the aboral ends (bases) of the first and second orals, 

 later becoming surrounded by the outermost processes of the first oral plates. 



The five orals stretch over the open vestibule anteriorly and are movable; the 

 five lappets in which they lie can be turned outward or inward, in the latter case 

 closing the vestibular entrance. 



The basals and orals are now so placed that the first basal and first oral, 

 directly superposed, lie in the interradius bounded by radii I and V, and the 

 fifth in the interradius bounded by radii V and IV. 



The basals lie beneath the ectodermal layer, and almost on the parietal layer 

 of the aboral ccelome. 



A weak layer of ectoderm, composed of mostly spindle-shaped cells standing 

 at right angles to the surface, also covers the orals. Apparently with the increas- 

 ing thickness of the orals all the mesenchyme cells of this region are drawn into 

 their compass to contribute to the calcareous secretion, or possibly to form loose 

 connective tissue filling the spaces in the calcareous meshwork. 



The individual plates increase in area through the formation of rods along 

 their borders which fork and join with the neighboring rods; and also the whole 

 meshwork thickens so that the spaces, at first broad, become narrower, and their 

 polygonal form rounds itself off more and more until the plates finally acquire a 

 sievelike appearance. 



The orals and basals do not always grow at the same rate; sometimes one 

 series, sometimes the otlier, is more extensively developed ; but very often also the 

 difference is negligible. 



The infrabasals, five or four, very rarely three, in number, are disposed, not 

 quite regularly spaced, in a circle of relatively small diameter about, or just 

 beyond, the oral (posterior) end of the proximal columnal. They are obliquely 

 placed in such a way that their aboral ends lie nearer the major axis than the oral. 

 But the grade of the obliquity varies in different individuals, as well as in the 

 different plates in the same series. They always, however, lie more horizontally 

 than the basals. 



In the younger stages the individual plates in this series are usually of very 

 different sizes. In the rare cases where there are three only the central one is very 

 small and the two lateral very large, suggesting the possibility that the latter have 

 each arisen through the fusion of two originally separate beginnings. 



