BLASTOIDEA. 299 



closely approximated hexagonal plates. ISTo aperture in the 

 test has been observed, and the plates are deeply concave, 

 each with an inward projection, which leaves a correspond- 

 ing pit in the cast. 



Various views have been held as to the affinities of all 

 these bodies, but the most probable conjecture, as before 

 remarked, is that they are allied to Receptaculites, and are 

 therefore large and aberrant forms of Rhizopoda. They are 

 nearly related to one another, and may prove all to belong 

 to a single generic type. All are found in the Silurian 

 rocks, and the only one which survives into the Devonian 

 is Splicer ospongia. 



ORDER VI. BLASTOIDEA. 



The Blastoidea or " Pentremites " are JEchinodermata in 

 which the body is enclosed in an armour of closely -fitting 

 calcareous plates, and is fixed to some foreign object by a 

 slender column. From the summit of the calyx radiate five 

 areas which are grooved longitudinally, and striated across, 

 and which carry a row of jointed pinnulce on each side. No 

 free arms; reproductive organs contained within the calyx; 

 respiratory apparatus of the type of the hydrospires of the 

 Cystideans. 



The Blastoidea in many respects closely resemble the 

 Crinoids, but are at once separated from them by the total 

 want of free arms, as well as by the structure of the calyx. 

 They are also nearly allied to those Cystideans which have 

 a definitely -constructed calyx and recumbent arms ; but 

 they are separated by the peculiar arrangement of the caly- 

 ciiie plates, and by the constant association of the respiratory 

 pores and tubes (" hydrospires ") with the arms. 



Taking Pentremites as the type of the order, we find the 

 body or " calyx " to be of a more or less globose, ovoid, or 

 pyriform shape, supported upon a short, jointed column, and 

 inclosed in an integument of articulated calcareous plates, 

 which are few in number, and have a definite form and 

 arrangement. The lowest plates of the cup (figs. 177, 178, A 

 and B) are three " basals," one small and quadrangular, the 



