SYSTEMATIC PART. 151 
compound nature of the radials in the earlier forms, are other primitive 
features characteristic of this group. 
The Fistudata, 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 z, 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 CrrvomeEa 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 Crrnoiwera 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, 
