HAMBACH— ANATOMY OF THE GENUS PENTREMITES, ETC. I5I 



and ambulacral fields (see Plate «:, Fig. ii & 12). Supposing 

 the column to be absent, we would have an analogue of an Echi- 

 nus, except that mouth and vent are placed, together with the 

 ovarian openings, on the dorsal part of the shell instead of being 

 on the ventral side as in true Echinoids. 



The Pinnulce* — T\-\Q pinnulas are little thread-like appenda- 

 ges, nearly as long or a little longer than the ambulacral field. 

 They are composed of small alternate pieces about as long as 

 wide, and situated along the lateral margin of the ambulacral 

 field, articulating with the flattened surface of the retreating angle 

 of the zigzag plicated integument, so that the poral openings 

 remain free between each two pinnul^e. They are not placed on 

 the po] al openings as supposed by Dr. Roemer. These poral open- 

 ings, as it appears, must have remained free ; because we fre- 

 quently find the remnants of collapsed tentacles preserved in them, 

 the so-called supplementary poral pieces. 



The interior organs. — In the foregoing I have tried to de- 

 scribe the different parts composing the calcareous shell of the 

 animal, and will now, in like manner, proceed to the description 

 of the softer parts or organs which were protected by the calca- 

 reous shell, though both are so intimately connected with each 

 other that one could not exist without the other. If we examine 

 these parts, which, in some cases, are very beautifully preserved, 

 we shall see that they are placed below or above each other ; and 

 if we commence with that portion which is placed immediately 

 under the lancet piece of the ambulacral field (see Plate a. Fig. 

 9 «), we will find here a longitudinal duct or vessel resting in 

 the concave furrow of the lancet piece, and running from the 

 apex of the ambulacral field to the summit, where it connects 

 with a circular duct (oesophageal ring?) surrounding, on the inte- 

 rior side, the central orifice or annulus centralis. This I have 

 been so fortunate as to obtain entire from a well-preserved speci- 

 men of Petitremites Norwoodi ; though, being probably com- 

 posed of a very fine and delicate tissue or membrane, it is de- 

 stroyed in most cases, and therefore very rarely observed. Next 



* The terms "pinnulce" and "tentacle" are here used in the sense of John Mueller; that 

 is, "pinnula;" for the more solid organs composed of calcareous matter, and "tentacle" for 

 the softer and membranaceous ones, sucli as occupy the pores of the ambulacral field in 

 Echinoderms. 



