ON THE ECHINODERMATA. 
265 
ON THE ECHIN0DEEM4TA * 
BY DR. T. WRIGHT, F.R.S. 
The EcJunodermata are highly organised animals, for the most part 
covered with a coriaceous or calcareous skeleton, and their surface 
armed with numerous spines, which aid in locomotion, and serve as 
defensive instruments. They have a complicated system of aquiferous 
canals, connected with the motion of their sucking feet, which, in the 
sea urchins, escape through holes in the shell, and in the starfishes, 
through intervals between the plates. They formed the highest 
group of Cuvier’s Kadiata, but are far in advance in their organisation 
of any of the singular animals with which they have been classed. 
As this is an elementary lesson, I shall take the leading orders 
in succession, commencing with the lowest form, and ascending 
gradually to the highest. We divide the Echinoderms into six orders: 
1, Holothuridea; 2, Echinoidea; 3, Asteroidea; 4, Ophiuroidea; 5, Cystoidea; 
6, Crinoidea. 
Crinoidea .—The name is derived from the resemblance which some 
of the fossil forms of this order have to the flower of a lily, hence stone 
lilies, from their infolded rays, resembling the petals of that flower. 
They have a body more or less spherical, supported upon a jointed 
stem, as in this Rhizocriims Lofotensis, discovered by Sars near the 
Lofoden Islands, 100-300 fathoms deep. The cup-like calyx is formed of 
close-fitting calcareous plates, varying in number in the different 
genera, and investing the surface like a coat of mail. The calyx is 
provided with five solid arms, which are independent of the visceral 
cavity, and are adapted for prehension in seizing their prey. They 
have a mouth intestine and vent distinct; no retractile suckers, and 
the ovaries open by special apertures at the base of the arms. Their 
skeleton is complicated and composed of many plates closely joined 
together; the number and arrangement of the elements are determinate 
in the different families, the multiples of five being the numbers 
which predominate ; the central part of the body is supported on a 
long-jointed stem, which is sometimes rooted to the bed of the sea, or 
coiled up as a portable support. The mouth is central and prominent, 
and the vent opens near its side. The arms are mostly ramose and 
multi-articulate, and when expanded form a net-like structure of con¬ 
siderable dimensions. The mouth is always placed upwards, so that the 
normal position of a Crinoid is the reverse of the starfish. In our 
present seas we have Pentacrinus Caput- MeAluscB, from the seas of the 
Antilles, Rhizocrinus Lofotensis, from the Lofoden Islands, Bathycrinus 
Aldrichianus, and Bathyocrinus Bethellianus, found in 1,850 fathoms 
water, during Challenger expedition ; 1° 47' W. long., 24° 26' W., 
in a bottom of globigerina ooze. We shall see presently that the 
Crinoids played a wonderful part in the ancient seas of the world. 
* Read before the Cheltenham Natural Science Society. 
