ECHINODERMA. 9 
appear at the outer edges of the ambulacral plates. The inter- 
ambulacral plates, which generally form a wider area, are not 
perforated. The perforated plates (pore-plates of Agassiz) are 
primarily single, but, with the exception of the Cidaride, they fuse 
into somewhat larger plates ; in a number, hence grouped together 
as the Triplechinide, there are always three pairs of pores on the 
edge of a well-developed secondary plate ; but there may be more, 
as in Strongylocentrotus. Though subject to some variation, the 
number of pairs of pores in a plate is a character of importance in 
discriminating species. 
In the Cidarids the membrane which bounds the mouth is heavily 
laden with both radial and interradial scales; in the Echinothurids 
only the perforate, radial scales are carried on to the buccal mem- 
brane, and in the rest of the Echinoidea regularia there are only 
five pairs of radial ossicles, whence they have been distinguished as 
decalepid from the others, which are polylepid. 
In the Crinoidea the calycinal plates are of functional importance, 
for they form a cup in which are contained the chief viscera of the 
body. In an early stage, if not throughout life, this cup is placed 
at the top of a stem, the basal joint of which is the central plate of 
the calycinal area; the centre of the cup of the free unstalked adult 
Antedon is formed by a modified plate which is called the centro- 
dorsal; the interradial circlet forms a small “ rosette-plate ” which 
is hidden from view, and there are three circlets of radial plates. 
The centrodorsal carries a number of jointed cirri or calcareous 
grappling-lines by which the creature temporarily fixes itself; if 
these fall off, their presence is still indicated by the cirrus-sockets 
on the centrodorsal. After the third radial there is one, or there 
may be more divisions into arms, the number of which in some 
foreign species approaches one hundred. There is also another 
dichotomous division which is alternately incomplete on either side: 
in this way there is formed on each side a series of minute arms, 
which hang down by the arm proper; these jointed structures are 
known as pinnules; they bear the gonads. The arms are made up 
of joints connected with one another by ligament; sometimes the 
ligament almost altogether disappears, and two adjoining ossicles 
are only separated from one another by what looks like a fine dotted 
line ; such apposition of ossicles is called a syzygy. The joint next 
below a division of the arms is known as an awillary. 
Madreporite.—On the upper surface of a Starfish a grooved scar- 
like plate will easily be detected among the reticulation of the inter- 
mediate plates; if this be examined with a lens, it will be seen to 
be minutely perforated; if its relations are studied, it will be found 
to be connected with a tube (“ stone-canal”) which runs vertically 
downwards and becomes connected with the water-vascular circle. 
This plate is known as the madreporite (see fig. p. 2). In the 
regular Echinoidea it always occupies one of the interradials (genital 
plates), and an Echinoid should always be described with this plate 
lying forwards and to the north-east. In many irregular Hchinoids 
