Prof, S. Lovdn on the Structure of the Echinoidea. 441 



it constantly remains attached by that opposite to vertical 

 plate 3, another pair of rounded pieces appears in the enlarged 

 interspace, and another near vertical plates 2 and 4 ; and, as 

 many such are added, gradually becoming smaller and smaller, 

 their number is greatly increased ; while the original central 

 disk, which itself does not grow much, is still long recogniza- 

 ble by its position and size. Finally the periproctium is 

 eroded into a large oval apertm*e ; the calcareous pieces which 

 closely fill its covering membrane become very numerous, and 

 the surrounding plates strongly eroded. The anal aperture, 

 which is not completed until this augmentation of the calca- 

 reous pieces in the skin is considerably advanced, is always 

 situated more or less excentrically in the apex of the membrane, 

 which gradually rises conically, and normally in the direction 

 of ambulacrum I. ; and the oval periproctium generally has its 

 longest diameter in the same direction from interradium 3 to 

 ambulacrum I., the same that is the longitudinal axis of the 

 test in Echinometra, and in relation to which the peristomial 

 plates of the interradia are symmetrically arranged in the 

 Latistellse. 



This formation of hard pieces of calcareous network occurring 

 in the central space, within the circle of the vertical plates, and 

 which, in the earliest stage, gives origin to a single disk, but 

 afterwards, during growth, divides itself regularly into different 

 centres for the production of numerous, free, smaller and 

 smaller pieces, agreeing in their texture with the first disk — the 

 whole of this structure, although in close connexion with the 

 appearance of the anus, belongs nevertheless not to its deve- 

 lopment but to that of the dermal skeleton ; and the complex 

 of hard parts which originates therefrom is an independent 

 part of the latter. It recurs in the Salenidse, not early broken 

 up into different small parts, but constant, coherent, and solid, 

 in the pentagonal disk which here regularly occupies the 

 central space. At the appearance of the anal tube it is partly 

 eroded by absorption in its posterior margin, but still more the 

 vertical plates lying behind, in Heterosalenia and Salenia, 1 

 and 5, in the normal direction towards ambulacrum I., — in 

 Acrosalenia^ GoniophoruSy and Peltastes only plate 5, as has 

 been explained by Cotteau, who was the first that correctly 

 oriented both these and all other forms of Echinidse, in the 

 manner here confirmed. In these genera it is not an added 

 supernumerary plate, but a normal part of the skeleton, which 

 in them retains during the whole life of the animal its original 

 form but little altered by the intrusion of the anal tube ; whilst 

 in other Echinidge it is very early changed into a flexible 

 covering, or, as in Diadema^ entirely disappears. It seems 



Ann. & Mag. N. Hist. Ser.4. Vol.x. 31 



