74 SVEN LOVEN, ON THE ECHINOIDEA DESCRIBED BY LINN^EUS. 



and its explication to the transformations observed in the 

 Strongvlocentrus dröbakensis, showing how at first the peri- 

 proct is distant from the radials, how it widens and encroaches 

 npon the surronndings, how in its backward movement it erodes 

 the costals 1 and 5, imtill it at last attains the radial I, then 

 the V, while the IV is approached and the II and III are 

 untouched, and how at the same time the anal membrane 

 expands and its ossicles become more numerous, while the 

 excretory opening recedes towards its margin in the direction 

 of I and 5. It remains to examine this process in other genera 

 of the Echinidae. At present it mav be assumed that in this 

 genus and most others a periproct not approaching or attaining 

 the radials I and V, an anal membrane with few and relati- 

 velj large ossicles, and a sub-central excretory opening, denote 

 the not adult or the young state, while the adult may be 

 known from a periproct eroding the two biviary radials, and 

 even nearing the IV, by a multipartite anal membrane, and a 

 posterior excretory opening. It is clearly not safe to use as 

 specific characters the difFerent stages of development coming 

 into view between these extremes. 



