164 NEW YORK STATE MUSEUM 



each suture. To apply the term rhomb to the triangular hydrospire 

 so formed is somewhat of a misnomer but the more serious error 

 lies in using the same term for types of structures so different in 

 their ontogeny. 



In " Paleozoic Fossils," volume II, part I, page 96, Billings pro- 

 posed the term hydrospires for all specialized respiratory areas in 

 echinoderms and on page 99 made the term include even the epithecal 

 respiratory areas of Paleocystis, for he called these " external 

 hydrospires.^ " It would be conducive of clear thinking, however, 

 if we could limit the use of the term " hydrospires " to specialized 

 folds of the outer body wall which became subthecal in position, for 

 if we may speak of the epithecal papulae of Palaeocystis or of 

 Palaeocrinus as " hydrospires " we might with equal propriety use 

 the same term for the papulae of asteroids. 



Restricting then the term hydrospires as suggested we still need 

 special names for special forms, as witness the term " multidisjunct 

 pectinirhombs," but such terms should be coined to express true 

 ontogenic differences rather than to denote an external similarity in 

 appearance due to convergence in development. For example, the 

 " triangular pectinirhombs " of Porocrinus might be called goni- 

 hydrospires, but as in all types of echinoderm respiration water is 

 the medium through which oxygen is received, we might shorten the 

 term, without loss, to goniospires. This name would be appropriate 

 not only because of the position of these hydrospires at plate corners, 

 but also because the folds themselves are bent abruptly through an 

 angle of about 60 degrees as they cross the sutures. Hydrospires 

 of the Glyptocystis or Pleurocystis type, from their position on plate 

 sides or margins, could be called craspedvspires. These two types 

 are alike in that their function is purely respiratory and their posi- 

 tion between regular thecal plates. 



The hydrospires of Blastoidocrinus and the Eublastoidea are of 

 a very different type, for these are " between thecal plates associated 

 with food grooves " (see N. Y. State Museum bulletin 149, page 

 198) and they perform the additional function of draining away 

 the surplus water drawn in through the process of food getting. 

 These are flowing hydrospires, and the water is changed in them 

 not simply by means of ciliary motion in their external folds, 



1 This pioneer work of Billings I had not seen before the publication of 

 my 191 1 paper dealing particularly with respiration in the genera Blastoidoc- 

 rinus, Palaeocrinus and Palaeocystis : " Studies of Some Early Siluric 

 Pelmatozoa " in N. Y. State Museum Bulletin 149. 



