SYSTEMATIC PART. 151 



compound nature of the radials in the earlier forms, are other primitive 

 features characteristic of this group. 



The Fistulata, in a general way, agree in the structure of the dorsal cup 

 with the Larviformia. But while in the latter the disk is simple in the 

 extreme, that of the Fistulata is highly specialized by means of the extrava- 

 gant development of the posterior interradius, forming a tube or sac which 

 often is drawn out to the full length of the arms. The sac rests upon the 

 radials or is supported by the anal x, and frequently — not always — con- 

 tains the anus. Owing to the large size of this sac, and the disturbances 

 thereby produced, the whole calyx acquired a marked irregularity, which 

 was still further increased in species with compound radials, so that the 

 pentamerous symmetry, and the bilateral also, were often supplanted by 

 asymmetry. The other interambulacral areas are but feebly developed, and 

 are pushed over toward the anterior side. The anal opening is situated 

 near the top of the sac, or at some place along its anterior side, and some- 

 times in front of the sac close to the mouth. The sac, at least in some 

 groups, is pierced by pores, which we think had respiratory functions, 

 similar to the water pores of recent Crinoids, in which also they are more 

 profusely distributed at the posterior area than at any of the four other sides. 

 In some groups, in which the sac is not perforated, we discovered upon the 

 disk, directly behind the mouth, a poriferous plate, a sort of madreporite, 

 which probably performed the same functions as the pores in the others. 



The Ceinoidea Camerata have a large number of supplementary pieces, 

 which are distributed almost equally throughout the five sides of the calyx, 

 by means of which the proximal arm plates for some distance are incor- 

 porated into the calyx, thereby increasing the capacity of the visceral cavity. 

 All plates of the calyx are heavy and immovable, being united by close 

 suture. The symmetry of the dorsal cup is either strictly pentamerous or 

 sub-bilateral; we never find that asymmetry which is so characteristic of 

 some of the Fistulata. The base is monocyclic or dicyclic, and mouth and 

 food grooves are perfectly closed. 



The Crinoidea Articulata agree with the Camerata in that their lower 

 brachials take part in the calyx, and help to enclose the visceral mass ; but 

 their plates from the radials up are movable. The incorporation of the 

 brachials took place by lateral union among themselves, by the introduction 

 of supplementary plates, or by means of an incrusted or naked skin; and 

 mouth and food grooves are exposed upon the disk. The base is dicyclic, 



