ECHINODERMATA : BLASTOIDEA. 



In some Cystoidea there were no arms, properly speaking, 

 but only small pinnulae. In a second section two arms were 

 present, but these were bent backwards, and were immovably 

 soldered down to the body. In one single species (Comaro- 

 cystites punctatus, Billings), the development has gone further, 

 the arms being free, and provided with lateral pinnulae, as in 

 the true Crinoids. 



Many Cystideans are likewise provided with a system of 

 pores or fissures, penetrating the plates of the body, and usu- 

 ally arranged in definite groups. These groups are termed 

 " pectinated rhombs," but their exact function is doubtful. By 

 Mr Billings, however, they are believed, and apparently with 

 good reason, to have admitted water to the body-cavity, and 

 to have thereby subserved a respiratory function ; though the 

 recent researches of Ludwig on the genital glands of the 

 Ophiuroids would render it not improbable that they were 

 also connected with the function of reproduction. 



ORDER BLASTOIDEA. Body enclosed in an armour of closely 

 fitting calcareous plates, attached to some foreign body by a slender 

 stem. From the summit of the calyx radiate five transversely 

 striated and longitudinally grooved areas, which carry a row of 

 jointed pinnulce on each side. 



The members of this order, like those of the preceding, are 

 all extinct, and are entirely confined to the Palaeozoic period. 

 The body (fig. 108, a) was fixed to the bottom of the sea by 



Fig. 108. Morphology of Blastoidea. a Pentremites pyriformis, viewed sideways, 

 showing a portion of the column ; b Summit of the calyx of Pentremites cervinus, 

 showing the pseud-ambulacral areas and the apical apertures ; c Side view of Grana- 

 tocrinus tnelonoides ', d Summit of Granatocrinus neglectus. (Figs, a and b are 

 of the natural size; c and d are slightly enlarged.) After Hall, and Meek and 

 Worthen. 



means of a short, jointed pedicle ; it was globular or oval in 

 shape, and composed of solid polygonal calcareous plates, 

 firmly united together, and arranged in five inter-ambulacral 

 and as many ambulacral areas. (These ambulacral areas are 



