REPORT OX THE ECHINOIDEA. 



73 



the actinal membrane are pierced for the passage of amlmlacral tentacles, new plates for 

 this membrane are formed by becoming detached from the ambulacral zones. It is 

 readily seen on examining a large Astheiiosoma how the small ambulacral j)lates of the pori- 

 ferous zone become isolated and gradually increase in size laterally, until they eventually 

 meet the corresponding plates from the adjoining ambulacral zones; and thus, in spite of 

 the increasing size of the actinal opening with advancing age, it is kept covered by the 

 newly-formed plates detached from the edge of the test at the point of contact with the 

 actinal membrane. These plates extend on one side towards the median line of the 

 ambulacral zone, and in the other direction meet on the median line of the interam- 

 bulacral zone. In the younger stages there are plates in the extension of both the 

 ambulacral and interambulacral areas. 



Lovdn and Ludwig have shown that the imbrications of the plates so characteristic of 

 the Palaeozoic Eehinoidea is not completely lost, even in Spatangoids; and from the peculiar 

 mode of growth of the plates, regularly concentric round a nucleus, a thinner edge is 

 formed, which maintains by its encroachment on adjoining plates a considerable degree of 

 mobility in the tests of the globular Spatangoids during their growth. And when we go 

 back to the very earliest stages of growth of the plates composing the test of Echinids, 

 when they are made up of mere Y-shaped rods, we can readily see that the lapping of the 

 coronal plates is a feature very characteristic of all Echinoid structures from the very 

 nature of the basis of the calcareous plates composing the test ; whether it be in a 

 Palaeozoic Echinus, a Cidaris, a Spatangoid even, or a Crinoid, a Starfish, or a Holo- 

 thurian, it is found occurring in all the plates. 



The gills pass as in the Diadematidse proper between the edge of the plates of the 

 test jjroper and the imbricating plates of the actinal membrane, though the openings 

 through which they pass can scarcely be called cuts ; they are small indentations, the 

 result of the space left between the curved edge of the last interambulacral plate, and 

 the next plate of the actinal membrane, or of the notch formed by the overlapping of the 

 extremity of the last plate over the side of the next coronal plate. The gills appear at a 

 very early stage; in the youngest specimens, they are mere digits, or a simple fork in the 

 smallest specimens I have examined. The gills become quite prominent in some of the 

 species (see PL XIX." fig. 1, Asthenosoma tessellatum). New plates of the anal system, 

 on the contrary, ajapear to form next to the anal opening, and are gradually pushed away 

 towards the genital plates, though evidently additional plates are also formed by the 

 .splitting of the older and larger plates, especially those adjoining the abactinal extremity 

 of the interambulacral area. 



Judging from the large size of the genital openings and the large size of the eggs in 

 one of the species, this group of Sea-urchins is probably viviparous; and we find here also, 

 in the great distance at which the genital openings are placed from the anal system, some- 



(ZOOL. CHALL. EXP. — PART IX. — 1881.) I 10 



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