THE AMBULACRUM OF THE CORONA. 61 



V 



A similar condition of fan-shaped plates proximally, which are not so shaped distally, is seen 

 in Melonechinus multiporus (Plate 5G, figs. 4, 5) and M. giganteus (Plate 61, figs. 5-9). 

 Comparable differences between the interior and exterior sides of the same plates are seen in 

 interambulacral, ocular, and genital plates, as described in the consideration of these parts (pp. 

 75, 96, 172). All this shows that in the study of specimens and the description of species, one 

 must bear in mind whether the specimen represents the internal or external characters of the test. 



Another feature noticed on the inside of ambulacral plates is elevated nodose or spinose 

 projections that occur in some Echini. These were first seen in the fossil Hyattechinus (Plate 

 24, fig. 6), where low, knob-like or spinose elevations exist between th(> inner pore and the 

 middle of the area. Alexander Agassiz (1904, p. 31) has described spines extending into the 

 body from the inner face of the peristomal ambulacral jilates of Porocidaris cohosi, but I believe 

 they have not been noticed before in coronal ambulacral ])lates. In Phyllacanthus (Plate 3, 

 fig. 12) near to the ventral border of the test, there are small spines between the inner pores 

 and the middle of the area; these seem to be the equivalent of those occurring in the Palaeozoic 

 Hyattechinus. Close to the peristomal border these spines increase in height, arch over and 

 then fuse in a continuous ridge (text-fig. 224, p. 193). Passing dorsally, in Phyllacanthus a 

 second series of spines occurs, one over each inner pore, and above the ventral area these 

 alone exist. In Eucidaris tribuloides I find a similar condition to that of Phyllacanthus, except 

 that there are commonly three spinules over the inner pore (Plate 3, fig. 13). tSuch spinose 

 growths are most delicate and are destroyed by a touch so that they can only be seen in a test 

 cleaned without any brushing. They have not been noticed in any of the Centrechinoida. 



Closely associated with the ambulacrum are the spheridia described by Loven (1874), 

 which, as he showed, are characteristic of all modern Echini except the Cidaridae. If they had 

 existed in pits, as is so often the case, we might expect to find them in the Palaeozoic. The 

 fact of their absence in the primitive Cidaridae is an argument for the assumption that they 

 were also probably absent in Palaeozoic genera. Certainly they are not known. 



Pedicellariae have recently come into prominence through the critical studies of Mortensen 

 and others. It is of interest to note that tridentate pedicellariae occurred in the Palaeozoic, 

 as shown in Meekechinus elegans gen. et sp. nov. (Plate 76, figs. 8, 9). Except as shown in 

 the Jurassic Pelanechinus by Clroom (1887) I believe these are the first found fossil, and they 

 will probably always remain rarities. 



Special respiratory organs are also closely associated with the ambulacra. Charles Stewart 

 (1879) first described internal branchiae in Eucidaris iribuloides which lie dorsal to the lantern 

 and extend over the ambulacral areas radially from beneath the compasses and between these 

 and the braces. The same structures were described by Ludwig (1880) and by Prouho (1887) 

 in Cidaris papiUata. Prouho gave them the name of Stewart's organs, a convenient distinction 

 from the quite dilTerent external gills. I have seen them in dissections of Cidaris affinis, 



