294 Prof. S. Loven on the Structure of the EcTiinoided. 



the genera. What determines the issue of this removal is that 

 the pores of the entire plates have, even in comparison with 

 the size of the plates, a greater movement than those of the 

 half plates. Every pore which belongs to an entire primary 

 plate departs by degrees from its margin and approaches the 

 middle. Within every large plate this movement is strongest 

 in the first, adoral primaiy plate, and combined with a drawing 

 downwards ; in the last, aboral primary plate the same move- 

 ment occurs, although in a less degree. In the intermediate 

 half plates the shifting of the pores is nothing or almost im- 

 perceptible in the first, but more considerable and increasing 

 in the following ones. Consequently, if a large plate is com- 

 posed of a first entire primary plate, (1), three intermediate 

 plates, (2, 3, 4), and again an entire plate, (5), the first pore 

 moves far inwards nearly to the middle of the plate, the second 

 retains its original position, the third has drawn itself a very 

 little inwards, the fourth rather more, and the fifth still more. 

 But it is a consequence of this unequal movement that the 

 first pore no longer belongs to the original pore-arc, but has 

 separated therefrom and entered and completed a new, secon- 

 dary arc, the other members of which are constituted by the 

 pores of the preceding large plate, with the exception of the 

 first. The arcs of 3, 4, 5, 6, or 7 pores which characterize 

 Toxopneustes, and in which the number of pores is dependent 

 on the number of intermediate plates, are therefore always 

 counted from and including the second pore in one large plate, 

 to and including the first in the following plate. These altera- 

 tions of the ambulacra are represented in PI. XIV. figs. 2-8. 



In the peristome, even in individuals of small size, all order 

 seems to have disappeared in consequence of these shiftings. 

 This, however, is only apparently the case. A careful ex- 

 amination shows that every thing arranges itself in accordance 

 with the same law. 



The peristomial plates of series I. a-V. h present the fol- 

 lowing alterations. The rudimentary double pore (1), which 

 remains only as a notch in the very margin, moves gradually 

 past the middle of the first plate and becomes still more incon- 

 siderable ; for whilst the corona grows near its vertical pole, 

 some of its solid substance disappears in the margin of the 

 peristome, where its calcaceous deposit is slowly absorbed, 

 with the result that the pore-cup which was moving thither 

 becomes, as it were, eaten away, and loses a greater or less 

 portion of its wall. The perfect double pore (2) in tlie first 

 primary plate (1, i), Avhicli is an entire plate, moves, like this, 

 from the suture towards the middle, and also approaches the 

 margin, so that by degrees it loses a good deal of the wall 



