ECHINOMETRID^:. 365 



ECHINOMETRIDJE Gray. 



GENERAL CONSIDERATIONS. 



There can be little question that this family includes the most highly special- 

 ized of the recent regular Echini, for the elongation of one axis, when combined 

 with highly developed ambulacra, indicates an unusual complexity of structure. 

 And yet in the characters of the abactinal system and the globiferous pedicel- 

 lariae, the more specialized Echinidse, such as Tripneustes, are apparently more 

 advanced than any of the Echinometridae, and it is therefore merely a matter 

 of opinion whether Tripneustes or Heterocentrotus is considered the "highest" 

 of the regular Echini. They represent different lines of development and cannot 

 properly be considered competitors. Since, however, the ambulacra are held 

 to be the most important structures of an echinoid and the multiplication 

 of ambulacral elements the best evidence of increasing specialization, we do 

 not hesitate to rank the Echinometridse as the last and highest of the families 

 of Regulares. In addition to the characters already mentioned, Jackson (1912, 

 Mem. Boston Soc. Nat. Hist., VII) has brought out the interesting fact that 

 in the Echinometridse, ocular V is the first to become insert instead of ocular I 

 as in the Echinidse and Strongylocentrotidse. This seems a very significant 

 fact when taken in connection with the small size of the abactinal system. 



The family is confined to the tropics and is made up entirely of reef-dwelling, 

 littoral species. With the exception of the two West Indian Echinometras, 

 none of the family are found outside of the Indo-Pacific region. It is interesting 

 to note that with only two exceptions, there are no localities north or south of 

 30 latitude, where members of the family are known to occur. One of these 

 exceptions is the Bermuda Islands, which are north of 30 N., where Echinometra 

 lucunter is common, and the other exception is Lord Howe Island, which is 

 south of 30 S., where Echinometra mathcei occurs. The old record of the occur- 

 rence of the latter species at the Cape of Good Hope has not been confirmed by 

 later collecting and is doubtless erroneous, while the reported occurrence of an 

 Echinometra in New Zealand waters (Index Faunge Novse Zealandse, p. 288), 

 supposedly based on the collections of the "Challenger," is obviously erroneous, 

 as the "Challenger" collected no shallow water Echini whatever, in that region. 



