NIAGARA GROUP. 
221 
u small oblong vesicles resembling the pores, but never piercing through and reaching the 
u inner surface of the plate. These vesicles gradually lengthen, and at last unite into an elevated 
u ridge, occurring between the rows of pores where they are double; but, where the row is 
“ single, below it on the upper half of the plate, and above it on the lower half — another 
u very singular instance of symmetry of arrangement in this part. 55 
I believe I shall be able to show that this symmetry in the arrangement of these ridges with 
reference to the rows of pores on the exterior surface, has likewise some connection with the 
internal arrangement of the pores, and the organs connected therewith. I should first observe, 
however, that it is not always with age that this change takes place ; for some of the specimens 
exhibiting these radiating ridges most strongly, are below the medium size; while the largest 
specimens I have seen, though exhibiting in other respects every mark of age, are quite free 
from such ridges. It has been already remarked, that these pores communicate with the internal 
organization of the animal; which is shown on the examination of the inner surface of the 
plates, individually or collectively. The rows of pores are found penetrating the plate, from 
the openings on the inside of which proceed minute but very distinct grooves, extending to the 
edge of the plate : these grooves are margined by a thin elevated border, giving sometimes 
almost the appearance of a semicylindrical tube, proceeding from the pores to the margin of 
the plate, in relation to which they are always vertical. At the junction of the plates, these 
little grooves meet others coming from the opposite plate, which, being likewise vertical to 
the margin of the same, meet the adjoining ones in a continuous line, and we have a fascicle 
of grooves with parallel interspaces. In the living animal, the pores undoubtedly offered 
the means of protruding some organ necessary to the vitality and growth of its body. These 
filaments, or organs, of whatever nature, passed along the little groove extending to the margin 
of the plate, there meeting similar organs from the pores and grooves of the adjoining plate. 
As one plate became extended, the adjoining one was necessarily extended in a corresponding 
degree ; a new pore was added on each plate exterior to those already existing, and another 
filament or fleshy tentaculum protruded from each. In this way nearly the whole interior of 
the animal, from the base to the centre of the pelvic plates, was covered with fascicles of 
filaments extending across the joining edges of the plates, connecting them together, and un¬ 
doubtedly acting an important part in their formation and extension. Now when we examine 
the ridges which exist between the double rows of pores on the exterior, and those above the 
single rows on the lower half of the plate, and below the single rows on the upper half of the 
plate, we find this symmetrical arrangement having relation to the direction of the little grooves 
proceeding from the pores on the inner side of the plate. In all cases the external ridge has a 
corresponding one on the interior; and there is always an internal one, though none be visible 
externally. On the inner side of the plate, it will be found that where there is a double row of 
pores, that one row is directed, in the little groove, to the margin of the plate on the right; 
while the other goes to the left, leaving the ridge in the centre. Then, again, all the tenta¬ 
cular filaments from the single rows of pores on the upper half of the plate are directed to 
the upper margins of the plate, leaving a ridge below their inner opening, as well as one below 
