CLASS II CYSTOIDEA 181 
stride one another like the draw-tubes of a spy-glass. In other cases the upper 
part, and occasionally, indeed, the entire stem, is composed of vertical rows of 
alternating plates. These plates, as a rule (Dendrocystites), enclose a large 
central space, which may be regarded as a prolongation of the body-cavity. 
The Cystideans constitute the oldest and least specialised group of the 
Pelmatozoa. Appearing first in the Cambrian, they develop a great variety of 
forms in the Ordovician and Silurian, but become extinct before the close of 
the Permian. While their own ancestry is obscure, it is highly probable that 
from them have descended both the Crinoids and Blastoids. If, on the one 
hand, the Aristocystidae, Sphaerodontidae, Camarocystidae, and Echinosphaeritidae 
differ radically from the Crinoids in respect to their numerously and irregularly 
plated calyx, or as regards the feeble development or even total absence of 
their arms ; nevertheless, the Cryptocrinidae and the unfortunately ill-preserved 
Cambrian genus, Lichenoides, evince a striking similarity, especially as concerns 
the more or less regular arrangement of the calyx plates, and a certain approach 
to radiation. In fact, so far as the construction of the calyx alone is concerned, 
forms like Porocrinus and Hypocrinus may be assigned with equal propriety to 
either the Crinoids or Cystids. The presence of pectinated rhombs and 
calycine pores, however, and the porosity of the calyx plates, are characters 
which positively identify them as belonging to the latter class. 
If we can account for the derivation of Crinoids from the Cystideans on the 
supposition that the calyx plates of the latter gradually took on a more 
definite arrangement, while the loss of pores and pore-rhombs was counter- 
balanced by a stronger development of the arms and the stem; so, too, it is 
possible to explain the origin of the Blastoids from the same source. Although 
hydrospires are clearly wanting in the Cystids, nevertheless, other characters, 
such as the recumbent attitude of the arms upon the sides of the calyx, or 
their insertion in grooves on the ventral surface, as well as an approach to 
ambulacral areas in certain forms (Asteroblastus), predicate an intimate relation- 
ship with the Blastoids. Probably the most notable similarities are presented 
by the peculiarly modified families, Callocystidae and Agelacrinidae. Various 
attempts have been made to affirm a connection between Agelacrinus and the 
Asteroidea, and between Mesites and certain of the primordial Echinoidea (Cysto- 
cidaris) ; but such hypotheses are scarcely warranted, since they proceed from 
an over-valuation of mere external resemblances, which are in nowise indicative 
of kinship. 
The classification of the Cystoidea remains in a very unsatisfactory condition. 
This is owing in part to the comparative scarcity of material, as well as its 
frequently imperfect state of preservation; and in part to our insufficient 
understanding of many of the structural modifications. The classification of 
Johannes Miiller was based primarily upon the structure of the calyx plates, 
and three main groups were recognised :—Aporitidae, Diploporitidae, and Rhomli- 
feri. These divisions, however, embrace a number of very heterogeneous 
elements, and in nowise fulfil the requirements of a natural system. The 
classifications proposed by Barrande and Steinmann are still more inadequate. 
It is advisable, therefore, to follow the example of Neumayr and Bernard, and 
merely arrange the different genera into families ; the establishment of larger 
divisions being deferred until the group shall have been monographed. 
