152 TKANS. ST. LOUIS ACAD. SCIENCE. 



and directly under this duct we find an organ (Plate a, Fig. 9 b) 

 which is probably designed for the function of respiration ac- 

 cording to Billings, and termed "hydrospire" by him. It repre- 

 sents a membranaceous sack of peculiar construction, as it can 

 not very well be of calcareous lamellae, admitting Billings' defini- 

 tion to be a correct one. It occupies, longitudinally, half the 

 under surface of the ambulacral field, covering closely one side 

 of the duct above it, and connecting its lateral acute margin with 

 the poral fissure, whereas the other side of it — that is, the one 

 which faces the visceral cavity or the vertical axis of the calyx — 

 is folded in 2, 3,4, 5 or more plications according to the different 

 species, though these plicas often vary in number in the same 

 specimen. These foldings are so arranged that they represent an 

 unsymmetrical figure 8, of which the upper loop is larger than 

 the lower one. This sack commences also at the apex of the am- 

 bulacral field and runs to the summit, where each of the plicas 

 are resting in little grooves placed at the lateral expansions of the 

 deltoid pieces, and do not unite with the adjoining sack into one 

 tube, as supposed by some authors. As already stated, the acute 

 lateral margin is situated under the poral fissure from whence the 

 tentacles originate, leaving the interior of the calyx through the 

 poral openings, and forming in their collapsed state the supple- 

 mentary poral plates of Dr. Roemer. This can easily be observed 

 in good and suitable specimens by grinding ofi'the lateral margin 

 of an ambulacral field, where we will find that the interior cir- 

 cumference of a poral opening is lined with a membranaceous 

 integument. In such well-j^reserved specimens we may also ob- 

 serve that the upper loop of the plications surrounds, or that the 

 cavity produced by these folds is filled out by, a tube, which I 

 suppose to be the ovarian tube, and which has its outlet through 

 the ten openings surrounding the annulus centralis; explaining, 

 on tlie other hand, the necessity of the grooves for the support of 

 the hydrospiric plication, which by this arrangement are kept 

 from obstructing the free passage of the ovulum. It will be seen 

 from the whole arrangement that the central opening as well as 

 the surrounding ones could not well have been closed, as it 

 also contradicts the supposition that two of the hydrospires 

 were united into one tube ; because, if so, then this union could 

 only have been formed external to the cavity : for instance, in 



