166 ECHINODERMATA— PELMATOZOA phylüm iv 



open externally by slits piercing the radials and deltoids and running parallel 

 with the ambulacra. 



The functions of the hydrospires can only be surmised, but they are sup- 

 posed to have served for respiration : they correspond doubtless to the pec- 

 tinated rhombs and calycine pores of the Cystideans and to the respiratory 

 pores of some Crinoids. It is probable that water was admitted to the 

 hydrospire sacs through the marginal pores, and was discharged through the 

 spiracles. Roemer and Forbes have suggested that the hydrospires may 

 also have performed reproductive functions. Ludwig has called attention to 

 the resemblance between the genital bursae of Ophiuroids and the slit-like 

 spiracles in Orophocrinus ; his theory is that the hydrospires served both for 

 purposes of respiration and for the discharge of genital products, a view 

 which was also shared by Carpenter. 



The stem in Blastoids is preserved only in exceedingly rare instances. It 

 is round, provided with a small axial canal, and composed of short joints, 

 which apparently multiplied in a similar manner to that in the Crinoids. In 

 Orophocrinus and Pentremites it has been traced for a length of 15 cm. without 

 reaching the end; and in the latter form it has occasionally been found with a 

 few, comparatively heavy cirri. A f ew genera, like Eleutherocrinus, are stemless. 



It has frequently been claimed, owing to the superficial resemblance of 

 their ambulacral areas, that the Blastoids and Echinoids are mutually related ; 

 but such presumptions are founded upon a total misconception of the value 

 of external characters. The construction of the calyx, the presence of pin- 

 nules, and the stemmed condition, are features which identify them unmis- 

 takably as Pelmatozoa ; and their nearest relatives under this group are the 

 Cystideans. The parallelism between the ambulacral fields of the one class 

 and the recumbent arms, apparently soldered on to the calyx of the other, is 

 self-evident. The hydrospires of Blastoids correspond to the pore-rhombs of 

 Cystideans, as has already been remarked ; and the position of the mouth and 

 anus is the same in both types. The Blastoids constitute a peculiar, but, on 

 the whole, a very well-defined group, which is now regarded as of equal rank 

 with the Crinoids and Cystids. 



: The earlier forms occurring in the Ordovician are primitive, representing 

 transitions from ancestors of Cystid type, and having the characters of the 

 two groups intermingled in varying degrees. In one genus, Asterohlastus, the 

 presence of diplopores and lack of hydrospires are correlated with the presence 

 of the Blastoid ambulacrum together with its bordering pinnules, and more 

 strongly developed basals and radials. In another, Blastoidocrinus, the diplo- 

 pores are replaced by hydrospires, thus further strengthening a line of 

 development which becomes thoroughly established in the Silurian with the 

 genus Troostocrinus. 



Several genera are represented in the Devonian, both of Europe and 

 America. But the climacteric of Blastoid development takes place in the 

 Lower Carboniferous of North America ; some of the beds of the Kaskaskia 

 Group are densely charged with their remains, which, as a rule, are excellently 

 preserved. They occur sparsely in the Upper Carboniferous and Permian of 

 western America and the island of Timor, but above this horizon no traces of 

 Blastoids have as yet been discovered. Nineteen genera, comprising upward 

 of 120 species were recognised by Etheridge and Carpenter in their mono- 

 graph of 1886, and a few have been added since. 



