ECHINO DERMA. 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 Cidaridse, they fuse 

 into somewhat larger plates ; in a number, hence grouped together 

 as the Triplechinidse, 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 calvcinal 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 axillary. 



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 Echinoids 



