64 NATURAL HISTORY. [n. ZOOL. GAL. 
have the body covered with a leathery skin and strength¬ 
ened behind with five pairs of radiating ribs. The arms 
are compressed and furnished with a series of pores on each 
side ; the outer side of each pore is partly covered with a 
bundle of small spines arising from a common base. 
The arms of the Astrophyton (Cases 22 and 23) are 
round and repeatedly branched so as to end in very small 
flexible filaments, by which the animal attaches itself to 
marine bodies and strains its food from the surrounding 
water. The Euryale has square arms only branched at 
the tip. The Natalia has round simple tubercular ringed 
arms. The Laspalia only differs from the former in 
having elongated simple arms covered with a crustaceous 
coat. 
The remainder of the Echinodermata have a purse-like 
body, and the middle of their back is produced into an 
elongated stem, by which they are often attached to other 
bodies, or if they are free, the middle of their back is fur¬ 
nished with a bunch of hooked fibres, by which they attach 
themselves to corals, sea-weeds, &c.; and some of those 
which are free in their adult state are attached by a pedun¬ 
cle when very young. 
The Class Crinoidea (Case 24) are furnished with five 
arms, having a series of processes on each side like a 
feather. The arms are generally forked at their base, and 
often repeatedly subdivided. Their body is surrounded 
with hard shelly plates. One recent genus, Comatula , is 
free, but the other Pentacrinites and many of the fossil 
genera, as, Encrinites and Poteriocrinites , &c., have an 
elongated peduncle, which is furnished with claspers, like 
those on the back of the Comatula . 
The Class Sph&ronida are only found fossil and like 
the former, but they have no arms, and the body is more 
or less spherical, with a large aperture closed by five 
valves on the side of the vertex, between it and the 
costals, and they are only marked with scattered pores. 
The class Blasteroida (Case 24) appear to be nearly 
allied to the Crinoidea , but they chiefly differ in having no 
arms, and in their body being marked with five double 
series of holes like the ambulacra of Sea Eggs, as the 
genera Pentremites and Orbitremites . They are only 
found in a fossil state. 
