200 NEW YORK STATE MUSEUM 



tinirhonibs would be the result. There is still another type that 

 might arise. If the sac pore was divided by the growing corner of 

 the plate, one of the openings might remain on one suture and the 

 other opening on the neighboring suture. Subsequent plate exten- 

 sion might easily convert them into hydrospires and particularly so 

 if one of the openings should be so situated as to receive its water 

 supply by being associated with a covered food groove. The study 

 of cystidean plates will furnish the investigator with an almost 

 endless variety of plate structure and ornamentation, a. very large 

 part of which is the outward expression of respiratory structures 

 such as have here been designated. 



In those forms which developed recumbent food grooves which 

 in turn were protected by closely fitting covering plates, there would 

 be a mechanical factor tending to promote invagination between 

 certain members of the flooring plates of such a groove. This 

 mechanical factor would be pressure. The water accompanying the 

 minute organisms swept down each brachiole must exert some pres- 

 sure in the larger food-bearing streams of the food grooves. Invag- 

 inations that maintained exits outside of the pseudambulacral area, 

 of the type last outlined, examples of which may be seen in plate 3, 

 figures I and 2, would serve a double purpose. The water so 

 drained away would reduce the amount passing through the ali- 

 mentary canal and give more time for the digestion and absorption 

 of the food content. It would also allow an increase in number of 

 brachioles or an increase in activity and so secure a more abundant 

 food supply. On the other hand all water so drawn through the 

 pseudambulacral invaginations would be used for respiratory pur- 

 poses and the invaginated sheets would become extended into struc- 

 tures like the hydrospires of Blastoidocrinus or perhaps open into 

 each other and form structures like the more specialized hydrospires 

 of the Blastoidea. 



The respiration of Blastoidocrinus is more particularly treated 

 later in this paper and is adequately illustrated. An examination of 

 the matter there presented will serve to make this unique form of 

 respiration more clearly understood. Both forms of ectodermal 

 invagination here outlined serve to admit sea water beneath the test 

 and these forms of respiration may well be spoken of as endothecal. 



Evagination of the ectoderm and parietal layer of coclomic 

 epithelitnn or other membranes at the plate corners would give us 

 structures like those we are familiar with in podia and papulae. 

 Branchial vesicles so formed might come to lie along the sutures or 



