SUB-BRANCH I PELMATOZOA—CRINOIDEA 123 
Apart from this constitutional difference, the Echinoderms are distinguished 
from the Coelenterates by the presence of a true digestive canal, a vascular 
system, and water-vascular apparatus ; by a more perfectly developed nervous 
system ; and by an exclusively sexual mode of reproduction. 
The most striking difference consists clearly in the nature and composition 
of the exoskeleton (Figs. 217, 218). All of the plates, ossicles, spines, and 
other indurations are secreted in the integument (dermal connective tissue), 
and are all composed of microscopically small calcareous bodies embedded in 
organic matter. As these bodies are united to form perforated horizontal 
laminae, which are connected with one another by vertical pillars, the result is 
a minutely reticulated structure, highly characteristic of all Echinoderms. 
During fossilisation the interstices are commonly infiltrated with lime car- 
bonate, so that the whole structure is transformed into calcite, exhibiting un- 
mistakable rhombohedral cleavage. Each plate, joint, and spine of a sea-urchin, 
star-fish, or crinoid behaves mineralogically and optically like a single calcite 
crystal. 
All the Echinoderms are marine. In the classification proposed by Haeckel 
they are divided into three primary groups comprising the following classes :— 
((Class 1. Crinoidea. (Sea-lilies.) 
Sub-branch A. Pelmatozoa. ~- , 2. Cystoidea. Cystideans.) 
5) ‘ < v : 
» 3 Blastoidea. Blastoids.) 
; " fClass 1. Ophiuroidea (Brittle stars.) 
Sub-branch B. Asterozoa. Oe Cana: eS eneachen) 
: (Class 1. Echinoidea. Sea-urchins. 
Sub-branch C. Echinozoa. | OSS ee S ) 
\ 2. Holothurioidea. (Sea-cucumbers.) 

3% 
Sup-BrancH A. Pelmatozoa. Leuckart. 
The Pelmatozoa are Echinoderms which, during the whole or at least the 
early portion of their existence, are fixed by a jointed, flexible stalk, or are 
attached by the dorsal or aboral surface of the body. The principal viscera are 
enclosed in a bursiform, cup-shaped, or spherical test (calyx), which is composed 
of a system of calcareous plates ; and on the upper surface of which are situated 
both the mouth and anus, as well as the ambulacral or food grooves conducting 
to the mouth. Asa rule, jointed flexible arms spring from the distal ends of 
the ambulacral grooves around the margin of the calyx; sometimes, however, 
arms are wanting, the ambulacral areas being extended down the sides of the 
calyx, and beset on both sides with pinnules (Blastoidea). The inferior (dorsal, 
aboral) portion of the calyx is composed of a single or double series of basal 
plates, which either rest directly upon the stalk, or are grouped about a centro- 
dorsal plate. 
The Pelmatozoa are divided into three classes :—Crinoidea, Cystoidea, and 
Blastoidea. Of these, only the Crinoids are represented by a few existing 
genera; the others are wholly extinct, and are confined, moreover, to the 
Palaeozoic rocks. Although the Cystideans possess a less highly specialised 
organisation than. the others, and probably represent the common ancestral 
