388 COMPARATIVE ANATOMY CHAP. 



merely as parts of the skeleton, occur in all forms. They are found 

 in definite arrangement over the whole test, on both the ambulacral 

 and the interambulacral plates, but usually in greater numbers on the 

 latter than on the former. 



The spines are usually slender and pointed, but may also (e.g. the principal 

 spines of certain Cidaridce) be club-, egg-, plate-, or oar-shaped, etc. ; or again in 

 other cases they may look like fine setee. The skeleton of the spine shows the 

 same microscopic lattice-like structure which characterises all parts of the Echiuo- 

 derm skeleton. Transverse and longitudinal sections of the spines reveal specific 

 developments of this structure, the lattice-work being closer in some cases and 

 opener in others, so that a careful examination of the structure of an isolated spine, 

 taking into account certain possibilities of error, may suffice to determine the species 

 of a specimen. The spines are mostly solid, less frequently hollow (e.g. in the 

 Scutcllidcc). 



The spines are movably articulated with the test. Each spine 

 rises from a wart-like prominence of a test plate, which is called a 

 tubercle. 



Large, strong spines rise from large tubercles, and small spines from small 

 tubercles, so that an examination of the tubercles on the test of an Echinoid which 

 has lost its spines leads to some conclusions as to the nature of its former spinous 

 covering. The tests of the Clypcastroida and Spatangoida, for instance, have very 

 small tubercles, with which the small, inconspicuous, seta-like spines of these orders 

 correspond. The regular Echinoidea have powerful spines and large tubercles. In 

 the Cidaroida, especially, by the side of numerous small tubercles carrying small 

 s] lines, there occur, in the interradii, a smaller number of remarkably large tubercles, 

 which carry either very long and strong or else shorter but very massive spines, 

 (Fig. 336). 



Most spines are in some way ornamented, by ribs, thorns, etc. 



In describing the various parts which can be made out in a 

 spine, and in that portion of the test plate which surrounds its base, 

 we shall select a principal spine of Dnrocidaris pap-illatu (Fig. 337). 

 The spine consists of a shaft and a socket, the latter articulating 

 with the tubercle of the test plate. The shaft thins away near the 

 socket to form a neck which, again, is separated from the socket 

 by a projecting circular ridge or cushion. 



Each tubercle rises from a mound formed by the bulging of a 

 round, smooth area, the edge of which is surrounded by a circle of 

 smaller tubercles, which carry smaller spines and pedicellarice (Fig. 

 336). 



The socket, where it is in contact with the tubercle, has a pit, 

 and a similar pit is found at the centre of the tubercle itself. Within 

 these two corresponding pits runs an axial band consisting of elastic 

 fibres, which fastens the spine to the tubercle, and passes at its two 

 ends into the organic substance of the spine and tubercle. 



The base of the spine is surrounded by a double fibrous envelope. 

 The inner envelope consists of elastic fibres, the outer of muscle 



