154 ECHINODERMA. BLASTOIDEA 



or incipient spiracles. A further modification is seen 

 in Pentremites (fig. 65) in which, owing to the hydrospire 

 being pushed further into the cavity of the calyx, the folds 

 open into a common canal instead of into the space 

 between the summits of the folds and the overlying lancet- 

 plate and side-plates ; this canal opens orally by true 

 spiracles (fig. 62 B, s). The number of folds in each 

 hydrospire varies from one to nine. In a few primitive 

 types hydrospires are absent. In many Blastoids there 

 are five pairs of spiracles and an independent anus, but in 

 some genera {e.g. Pentremites) the pairs are confluent so 

 that only five spiracles are present, of which the posterior 

 encloses the anus. 



The ambulacra vary in width and length ; they may 

 be broad and petaloid or narrow and linear. In some 

 genera the alternate side-plates become squeezed towards 

 the outside of the ambulacrum ; here they form an outer 

 row, known as the outer side-plates, and are smaller than 

 the plates of the inner row. The side-plates may be 

 entirely at the sides of the lancet-plate (fig. 67), or they 

 may rest on it and partly, or even completely, conceal it 

 (fig. 66). The basals, radials, and deltoids vary considerably 

 in relative size — thus the deltoids may be very small (as 

 in Troostocrinns), or they may form a considerable part of 

 the calyx (as in Orhitremites). 



The most important characters of the Blastoidea as 

 a Class are found in the ambulacra and hydrospires, the 

 absence of true arms, the monocyclic base consisting of 

 three basals only, and the five incised radials. In a few 

 rare cases, hydrospires have been found to be present in 

 the Crinoidea {Carabocrinus, Hybocrinus). 



Codaster. Calyx in the form of an inverted cone or pyramid. 

 Basals forming a conical and usually deep cup ; radials large, with 



