282 ECHIXOMETRA. 



ECHINOMETRA. 



Echinometra Rond. 1504. De Piscib. (Bkeyn). 



Test thin, elongate, the longer axis making an acute angle with the 

 anterior axis, the madreporic body being placed on the right of this long 

 axis. Tubercles large, imperforate, not crenulate, slightly smaller in the 

 ambulacral area. Poriferous /.one moderately broad, pores arranged in very 

 prominent arcs of many pairs. Actinostome large, actinal cuts shallow and 

 often broad. Jaws very powerful, and auricles very massive. Spines long, 

 longitudinally striated, quite stout ; there are usually clusters of spines on 

 the ten buccal plates of the actinal membrane, — a feature which distinguishes 

 this genus from the true Strongylocentrotus, with which it is far more 

 closely related than has usually been considered to be the case. The ob- 

 liquity of the axis passing through the madreporic body and the opposite 

 ocular plate is an embryonic feature, arising from the closing up of the 

 open spiral which forms the lirst foundation of the Echinus in the Pluteus; 

 when this spiral becomes lirst closed, it is always eccentric, hut becomes 

 symmetrical in the other Echinidae, with the exception of the true Echino- 

 metradae where the obliquity remains through the adult stages. 



Nowhere among the young regular Echini have 1 found such great 

 changes in the shape and proportions of the test and spines as in Echi- 

 nometra, We frequently find specimens of the same size, where in one 

 case the outline is almost circular, the test flattened, covered with long 

 slender spines, while in the other the test is lobed. swollen, high, surmounted 

 by numerous short stout spines. These and all intermediate stages, compli- 

 cated by the greater or smaller number of primary tubercles, the arrange- 

 ment of the arcs of the poriferous zone undergoing changes exactly similar 

 to those described in Strongylocentrotus, are found existing in specimens of 

 very different size. This has given rise in a great measure to the confused 



•* Do 



synonymy attached to our most common species, and renders their identi- 

 fication, if based upon meagre material, almost hopeless. 



poriferous zone also is wider proportionally, the actinal opening is larger, and no trace of branchial cuts 

 can lie seen in specimens measuring r.».7 nim , the arrangement of the miliaries is more regular, and 

 they early have the two vertical rows of primary tubercles well developed. 



