194 HAWAIIAN AND OTHER PACIFIC ECHIXI. 



Sperosoma. 



Koehler, 1897. Zool. Anz. XX, p. 302. 

 Type-species, Sperosoma Grimaldii Koehler, 1. c. 



The general appearance of Sperosoma is much like that of Echinosoma, 

 the test being, as in that genus, thin and flexible with little difference 

 between the upper and lower sides. The spines and tubercles of the 

 actinal side are larger than those above and the primaries have well- 

 developed hoofs. The actinal tube-feet are in three distinct series and 

 have small sucking-discs or none. The sphajridia are as in Echinosoma. 

 Although the characteristic actinal ambulacra are very remarkable and 

 serve well to distinguish the genus, it must not be supposed that the separa- 

 tion of the primary ambulacral plate into an outer poriferous and an inner 

 non-poriferous part is a feature confined to Sperosoma. Many specimens of 

 Echinosoma tenue (and doubtless other members of that genus) show the 

 same phenomenon to a greater or less degree. It is quite common in 

 tenue to find the inner and outer halves of the primary plate separated by 

 a suture, even though narrowly in contact, and occasionally the two sec- 

 ondary elements nearly, if not quite, meet between them. This formation 

 of plates by resorption is one of the characters in which the EchinothuridiB 

 are most unique. The essential difference between Sperosoma and Echi- 

 nosoma in the structure of the ambulacrum is in the position of its component 

 parts ; thus although the primary plate of Echinosoma may be divided into 

 two parts, there are not four columns of plates in each half-ambulacrum, 

 for the upper (outer) secondary element lies above the outer half of the 

 primary plate and is more or less extensively a part of the interambulacral 

 margin of the ambulacrum. In Sperosoma, the outer halves of the primary 

 plates lie one above the other, broadly in contact, and forming the outer 

 of the four half-ambulacral columns. In Echinosoma this is not the case, 

 the outer column consisting of outer halves of primaries alternating with 

 the upper secondary plate elements, more or less irregularly. The existence 

 of a pair of median columns of imperforate ambulacral plates is a feature 

 in which Sperosoma is absolutely unique among all recent regular Echini, 

 but when we recognize the origin of the plates which compose them, we 

 see that it is not so much their presence, as the way in which they are 

 formed, that is really remarkable. In well-preserved specimens one can 



