ECHINUS. 303 
which lies beneath. Within this is a connective tissue layer 
of considerable relative thickness, in which the skeletal 
structures are developed. The calcareous substance of 
the test and spines takes the form of a more or less dense 
network, the meshes of which are everywhere filled with 
a protoplasmic ground substance. The ciliated epithelium 
of the body cavity may be in direct contact with the last- | 
named, or a layer of muscular fibres may appear between 
the two. 
The spines taper gradually from their bases to their 
more or less blunt tips, and are elegantly grooved from 
end to end. The base of each spine presents a saucer- 
shaped socket to the rounded tubercle of the test (Pl. L., 
fies. 1, 8, and 4) with which it is movably articulated. 
A capsular ligament binds the two together, and the move- 
ments of the spine are effected by the ftbres of a similar 
capsular muscle, which hes outside the hgament. When 
viewed in transverse section the calcareous meshwork of 
the spines presents an elaborate pattern. A transverse 
section of a decalcified spine (PI. I1., fig. 20) shows that 
the fusiform cells of the ectoderm are arranged in linear 
groups, coincident in position with the ridges which 
traverse the spine from base to tip. Their basal ends are 
continuous with processes of the cells which form the 
ground substance of the spine. Though the spines are 
mainly protective, they also assist locomotion. A healthy 
Echinus placed on a hard surface, out of water, at once 
begins to move, solely by the aid of the primary spines of 
the oral face of the test. 
Large numbers of pedicellariz are scattered over the 
surface of the test and peristome. They are modified 
spines, and are attached to the test by ball and socket 
joints. There are four well-marked forms, but all agree in 
