NO. 19.] ECHINODERMS OF CONNECTICUT. 87 



to set the spine stiffly in one direction. As long as the action 

 of this muscle lasts the spine can only be moved by tearing the 

 muscle. 



The test denuded of spines (Plate XXV) shows clearly the 

 regularity with which the spines are arranged, each tubercle indi- 

 cating the position of a spine. In Strongylocentrotus there are 

 numerous smaller secondary spines between the larger ones, as 

 shown by the smaller tubercles (Plate XXI). In this genus 

 there are several times as many spines as in Arbacia. 



Both the tube-feet and the spines assist in locomotion, as 

 described below. 



An examination of a living specimen shows five pairs of 

 meridional groups of very slender tube-feet, which are capable of 

 expanding to great length. The extremity of each tube-foot on 

 the oral surface has a sucker-like expansion, kept open by a per- 

 forated calcareous plate. The position of the tube-feet is indi- 

 cated in the denuded test (Plate XXV) by the ten series of plates 

 with minute perforations. These ten series of openings corre- 

 spond with the groups of tube-feet seen in the living animal, each 

 pair of openings representing the position of a single tube-foot. 



The series of ten rows of plates which bear the tube-feet con- 

 stitute the five ambulacral, or radial areas, there being two 

 rows of plates to each. Between the ambulacral areas are five 

 interambulacral, or interradial, areas of about equal extent 

 Each of these five areas likewise consists of two meridional rows 

 of plates (Fig. 13). 



The ambulacral areas may thus be seen to represent the oral 

 surfaces of the arms of the starfish, bent upward in a semicircle, 

 while the interambulacral areas correspond to the sides of the 



Explanation of Plate XXI. •Skeleton of Sea-urchin, Strongylocentrotus 



drobachiensis. (Natural size.) 



Figs. 1 to 3. Aboral, lateral, and oral surfaces with spines removed, 

 showing the regularly arranged rows of larger and smaller tubercles 

 which in life bear the primary and secondary spines respectively. Scat- 

 tered among the larger tubercles are numerous minute tubercles which 

 bear the miliary spines. The oblique rows of minute dots arranged in 

 pairs represent the pores for the tube-feet, and indicate the position of 

 the five double rows of plates of the ambulacral areas. Of about equal 

 extent are the five interambulacral areas. In Fig. 1 the central^ placed 

 periproct, consisting of many minute plates, is surrounded by a circle of 

 larger ocular and genital plates. In Fig. 3 the peristome and jaws have 

 been removed. 



