296 Prof. S. Loveii on the Structure of the Echinoidea. 



consequently furnished with eleven pores, so placed that they 

 may be counted as follows : — 1, 2 ; 3, 4, 5; 6, 7, 8 ; 9, 10, 11, 

 and, to complete the latter arc, 12 in the following large 

 plate 4, — that is to say, in groups of 2, 3, 3, 4, &c. The form 

 of this large composite plate has now become such that its 

 breadth stands to its height about in the relation of 1 : 0*7. In 

 the youngest specimen (fig. 2), in which the three plates are 

 quite distinct, the breadth is to their length, taken together, 

 as 1 : 2-25. 



Within the first coronal plates which belong to series I. Z>- 

 V. a corresponding changes take place, with only such differ- 

 ences as are due to the first large plate consisting only of two 

 primary plates. Here, also, the plates 1, 2, and 3 coalesce, 

 apparently almost earlier than in series I. a-V. h. The ternary 

 plate of the third order finally produced by coalescence has 

 then ten pores so arranged that they maybe counted 1, 2 ; 3, 

 4 ; 5, 6, 7 ; 8, 9, 10, and, to complete the arc, 11 in the next 

 large plate (4) ; consequently 2, 2, 3, 4, &c. It is by the 

 second number that we recognize the j)eristomial plates of 

 series I. h-Y. a ; it is there two, but three in series I. «-V. h ; and 

 this character is constant in the Latistella^, which may be 

 oriented by this means. The fourth arc, which here has four 

 pores, has only three in occasional individuals ; that is to say, 

 the third large plate has only one intermediate primary plate. 

 Some variability seems to prevail in this. 



In the Echinidas the tentacular pores are double pores. 

 Within an oval space or Cup bounded by a more or less 

 elevated wall open two straight passages, througli which 

 aquiferous ducts pass to the tentacle. Their openings on the 

 inside of the shell are considerably further apart than on the 

 outside. These passages consequently traverse the thickness 

 of the test in an oblique direction. If we compare the posi- 

 tion of the outer apertures with that of the inner ones in the 

 same plate of different ages and sizes, we find that the inner 

 ones do not change their position so much as the outer ones; 

 so that the passages which, in the younger specimens, take tlie 

 shortest course from the inside to the outside, gradually draw 

 away during growth in an oblique direction towards the 

 middle, in the same proportion as the outer apertures shift 

 their place. The movement which takes place in the sub- 

 stance of the plate is therefore not the same in its whole mass, 

 and has, the nearer we go to the outside, a preponderant 

 direction towards the median suture of the ambulacrum. 



Thus, in Toxojmeusfes drohachensis, do the ambulacra grow, 

 with constant alterations in the plates and pores ; but even in 

 the largest individuals the different character of the two dif- 



