18 



Trans. Acad. Sci. of St. Louis. 



membranous tubes and not bundles of tubes as stated by 

 various authors, and accompanied by incorrect illustrations. 

 They are located beneath the water duct and run parallel to 

 it from near the summit of the calyx to the apex of the 

 ambulacrum. The plications rest in little grooves of the 

 expanded lower deltoid portion, and are not united with the 

 adjoining one to form five bundles as described and illustrated 

 by Billings * and Ludwig.f These plicated tubes or hydro- 

 spires are of a peculiar construction. Each tube may be 

 regarded as a somewhat collapsed cylinder of which the upper 

 blade, i. e., the one nearest to the water duct, is smooth in its 

 whole length, whereas the underlying blade is folded into a 

 number of plications of no regularity. They vary in number 



Fig. 9. 



Transverse sections of ambulacral fields, to show abnormal 

 developments of the hydrospiric sacs: A, of Pentremites pyri- 

 formis; B, P. florealis; C, P. conoideus — about 20 times magnified 

 and drawn with the aid of the camera lucida. a, hydrospiric sac; 

 6, calcareous part of ambulacral field, i. e. lancet and poral 

 pieces. 



in the different species and vary very often in one and the 

 same specimen ; see Fig. 9 . At the outer margin where these two 

 blades meet, that is, where the upper smooth blade connects 

 with the lower plicated one, they run out into little thread-like 

 filaments or tentacles, giving the compressed cylinder a fringed 

 appearance on this side. They form the so much doubted 

 tentacles which protrude through the poral openings and form 



* Palaeozoic Fossils, Vol. II., Part 1, p. 102. 



t H. Ludwig. Morphologische Studien an Echinodermen, Band I., p. 289. 

 Taf. XXVIl,, Fig. 36, 37. 



