490 THE ANATOMY OF INVERTEBRATED ANIMALS. 



two ways : partly by addition to the circumference of the 

 existing plates, partly by the interpolation of new ambu- 

 lacral and interambulacral plates at the apical end of each 

 series, between it and the ocular or genital plate, as the case 

 may be. New plates are never added to the oral extremity 

 of the corona proper. 



The surface of the plates of the corona in the Echinidea 

 is covered with minute rounded elevations, or tubercles, to 

 which are articulated the spines so characteristic of the 

 group. The tubercle may be either simple or marked by a 

 central pit, into which, and a corresponding pit on the head 

 of the spine, a ligament of attachment is inserted. Further- 

 more, capsular muscular fibres connect the neck of the spine 

 with the base of the tubercle, and effect the varied move- 

 ments of which the organ is capable. The spines of the 

 Echinidea vary very much in form and size, from the close- 

 set, velvety pile of Scutella, or the delicate, spoon-shaped 

 blades of Amphidotus, to the long-pointed lances of Echinus 

 and the great clubs of Cidaris. Even on the same Eehino- 

 derm the spines may, as in the two latter genera, vary very 

 much in appearance ; and it becomes necessary to distinguish 

 those large ones which form a continuous series from one end 

 of an ambulacrum or interambulacrum to the other, as pri- 

 mary spines, from the other less complete secondary and 

 tertiary series. 



Loven 1 has drawn attention to the existence, in all the 

 Echinidea, except Cidaris, of certain minute spheroidal 

 bodies, rarely more than T ^ °f an mcn l or >g 5 which he terms 

 sphceridea. They occur upon the ambulacral plates, and es- 

 pecially upon those nearest the mouth. Each contains a cal- 

 careous and more or less dense and glassy skeleton, which 

 is articulated with a corresponding tubercle, as if it were a 

 miniature spine. In some genera, these sphceridea, to which 

 Loven ascribes a sensory function (probably auditory), are 

 sunk in fossae of the plate to which they are attached. 



Scattered among their spines, the Echinidea possess pedi- 

 cellarim, which are usually provided with long, slender stems, 

 terminating in oval heads, divided into three jaw-like pro- 

 cesses. The latter are strengthened by calcareous ossicles, 

 which articulate with an ossicle contained in the basal part 

 of the head, and a calcareous rod is usually developed in the 

 stem. 



1 " Etudes sur les Echino'ide^s," 1875. 



