TKTALOSTICHA. 



PETALOSTICHA. 



Suborder Petalosticha Haeckel, 1866. Generelle Morphol. (emend.) 



Among Spatangoids proper, the examination of young specimens shows 

 that they undergo great changes in outline during their growth ; the pos- 

 terior part of the test is especially subject to variation, the position of the 

 anus is exceedingly variable in one and the same species, the mouth is 

 not labiate in the young as in the adult, the peripetalous and lateral fascioles 

 do not change in their limits, but the subanal and anal fascioles are liable to 

 great modifications during their growth, and cannot be used as distinguish- 

 ing features of generic value, while the permanence of the peripetalous and 

 lateral fascioles is of great systematic value. The ambulacra! petaloids also 

 are greatly modified with age, generally becoming confluent, while in the 

 young they are remarkably distinct, and the pores not conjugated. 



Loven has already called attention to some of the most striking peculiari- 

 ties of Spatangus purpureus when young. To show the extreme care 

 which must be taken in our determination of genera among Spatangoids, I 

 have introduced figures of a small Spatangus purpureus (PI. XP. f. 19 - ..'..' ), 

 which is remarkable for its globular shape, but particularly interesting on 

 account of the structure of the abactinal part of the ambulacra (PI XP. 

 f. 19). The pores are as yet simple, not conjugate, resembling in every par- 

 ticular the simple pores of young Cassidulidae and of regular Echinidae, no 

 trace as yet being seen of the petaloid structure of this part of the ambu- 

 lacra. The ambulacra are in this young Spatangus identical in structure 

 with those of Holaster, and of other genera of Ananchytidae. 



The Cassiduloid-shaped mouth of young Spatangoids, as well as the exist- 

 ence of several Spatangoids, both fossil and recent, in which the mouth has 

 a similar structure, is a convincing proof of the correctness of uniting Cassi- 

 duloids and Spatangoids in the same suborder, though the name given to 

 them by Albin Gras, of " Irregular," is hardly what could be desired. 



The great number of Spatangoid genera established upon differences in 

 the subanal fasciole, the existence or absence of the anal branch, the depth 

 of the ambulacral grooves, the confluence or distinctness of the lateral am- 



