18 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



connection with the hydrocoel) it is evident that this is 

 already star-shaped ; that it is situated entirely on the 

 left side of the pluteus and orientated as in Echinoplutei 

 at this stage; and that the larval mouth will not take 

 part in the formation of the permanent mouth. Moreover, 

 as in Echinus again, there appears to be a membrane 

 closing in an amniotic cavity, into which the tube-feet 

 project. The plates of the Ophiuroid to which the larva 

 gives rise have developed to such an extent that the 

 ' terminal ' plates of the arms project from the body in 

 the form of a star, but (like the hydrocoel) instead of 

 hang in the horizontal plane of the pluteus, as is usually 

 the case at such an advanced stage of development, they 

 lie in a plane at right angles to this — the plane in which 

 they always make their first appearance in Ophioplutei. 

 The plates of the aboral surface have reached a 

 corresponding stage in their development, and also lie 

 roughly in the plane of their original formation. These 

 ' terminal ' and aboral plates do seem to have moved very 

 slightly towards the position assumed by those of other 

 Ophioplutei, but this movement, as shown by the position 

 of the middle of the central (aboral) plate, only amounts 

 to a rotation through about 12° instead of 90°. It is 

 unlikely, in view of the condition of the hydrocoel, thai 

 any further rotation will occur; but that some migration 

 of plates will take place is shown by the position of the 

 ' terminal ' plate corresponding to the forwardly directed 

 arm of the hydrocoel. This plate, as noted above, is still 

 in its original position (i.e., near the stomach of the 

 pluteus), whereas this arm of the hydrocoel extends 

 beyond the mouth. Thus the directness of development 

 found in Echinus, where the plates and soft parts arise 

 together in their final relative positions, has not been fully 

 acquired by this Ophiuroid. 



