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mentation of the whole colony. Among the known star-fishes now living, we have numerous 

 instances of such a centralisation manifested in the exterior. Between the forms provided 

 with long arms and particularly small discs (Asterias, Pedicellaster &c.) and the short-armed 

 star-fishes (Porania Pteraster) in which the arms are only insignificant processes of the power- 

 fully developed disc, we have all possible transitions. At last the arms appear to be en- 

 tirely wanting and only indicated by the pentagonal form of the disc (Goniaster) nay, in 

 the genus Culcita even the angulosity of the disc is indistinct; and we have here before us 

 a disc-like body in which only the 5 ambulacrals still indicate the original arms or per- 

 sons. The further change next consists in an incipient diminution of the antiambulacral 

 area, whereby the ambulacral area, originally confined to the ventral side, gains a constantly 

 greater and greater extension over the surface of the body; until at last the antiambulacral 

 area is still only visible as an insignificant space on the superior pole. In the periphery of 

 this space, there lie, in the regular Echinoids, the 5 peculiar ocellary plates with their pig- 

 mentary spots, exactly corresponding to the organs of sight situated in many star-fishes at 

 the extremity of the arms, whence again it appears that we have in the ocellary plates* the 

 counterpart of the extreme arm-joints of the star-fishes. In several Ecbinoids the body exhi- 

 bits moreover a very evident pentagonal form as indication of the original 5 arms. In like 

 manner we have among the Echinoids many forms in which the body is quite as disc-like as 

 in the star-fishes. Between these flat Echinoids and the high hemispherical, including the 

 pyramidal forms, we have all the transitions. If we imagine the high pyramidal form still 

 further developed in the same direction, it is not difficult to derive the cylindrical form, 

 which we find in the Holothurians. Here also the originally ventral (ambulacral I side occu- 

 pies the whole surface of the body; and of the antiambulacral area every trace has dis- 

 appeared. The peculiar development of the water-feet situated nearest to the mouth of the 

 Holothurians into capturing arms, is, like the complicated dental apparatus of the Echinoids, 

 of secondary origin; just as we can also see in the peculiar apparently bilateral symmetry 

 of the irregular Echinoids and of certain Holothurians (Psolus) only formations determined 

 by particular processes of adaptation. 



The other course of development, which likewise leads to considerable centralisation 

 of the cormus, is not properly speaking accompanied by any extensive concrescence of the 

 original individuals (arms, rays) while it is here essentially an internal transformation which from 

 the first takes place, a dislocation of the internal organic systems, whereby their principal 

 parts retract themselves, as it were, from their original place in the arms to the centre of 

 the colony, or to the disc, which hereby becomes the most essential and most organised 

 part; while the original individuals, the arms, are reduced to the rank of mere organs. The 

 organic system first acted on is also in this case the digestive system, the most important 

 really digesting sections of which in the star-fishes have still their original place in the 

 cavities of the arms as the so-called radial caeca, but which, in the course of development 

 here treated of, centralise themselves little by little in the cavity of the disc itself. This 



11* 



