484 Observations uri the 



and prominent, and occupy narrow, angular grooves be- 

 tween the median ridge of the lancet pieces and the edges 

 of the radial plates. The poral pieces of each side of the 

 field are adjacent at its lower end, but are separated at the 

 upper part of the field by a space about equal in width to 

 that occupied by each row of poral pieces. The poral 

 pieces projecting a little above the median ridge of the 

 lancet pieces, leaves a narrow groove along the centre of 

 each pseudambulacral field, which comes to a point at the 

 lower end. 



The whole central space between the summit tubes and 

 the anal aperture, which is rather small, is overlaid with 

 an integument of microscopic plates, entirely covering the 

 central aperture, passing out between the bases of the 

 tubes in a double series of plates, and was evidently con- 

 tinued far down the central grooves of the pseudambulacral 

 fields, causing the crenulated appearance of those parts 

 usually observed there, so that when perfect this integu- 

 ment was of a very sharply stellate form, the component 

 plates being closely connected throughout, and so small 

 that a space of half a line square contains over thirty of 

 them. They are prominent and distinct, but are some- 

 what irregular in size and arrangement. A fine hair-like 

 groove runs along the centre of each lancet piece, and 

 j^asses beneath the rays of the plated integument, the 

 plates of which arch over it. No specimens which we 

 have examined show a plated covering to the anal aper- 

 ture, but it can hardly be doubted that it really existed. 



One cannot pursue these detailed examinations of struc- 

 ture without awakening the inquiry, — what ofBce did 

 each of these various parts perform ? 



Of the summit apertures of Pentremitcs it has been 

 heretofore considered as certain that the central one was 

 the mouth, and that the others were ovarian apertures, the 

 larger of which was also the anal aperture. 



