THE ECHINOIDEA 



287 



(t l ), secondary (t~), and miliary (g). The primary tubercles are 

 the largest, and bear the primary spines ; each tubercle consists 

 of a rounded base or " boss " (b), on the centre of which is a small 

 pimple or " mamelon " (m). Around the base of each primary 

 tubercle is a smooth, level surface called the " scrobicule," which is 

 generally surrounded by a circle of granules called the "scro- 

 bicular circle." Scattered irregularly over the plates are the 

 smaller secondary tubercles which bear the secondary spines, and 

 between these are large numbers of smaller elevations known as 

 "miliary granules" (g). 



The function of these tubercles and granules is the support of 



Diagram of geinmifnrm pedicellaria of 

 Echinus aeutus (after Hamann). a.m, ad- 

 ductor muscles ; e.m, extensor muscles ; 

 f.m, flexor muscles ; gl, gland ; gl.c, 

 glandular epithelium ; s.o, stem ossicle. 



Fio. III. 



Appendages of Echinus. Transverse 

 section of spine (magnified). 



the appendages known as "spines" 

 and " pedicellariae." The Spines, 

 like the tubercles, are of three 

 sizes primary, secondary, and 

 tertiary the structure of each of 

 which is fundamentally the same. 

 Each consists of a long shaft, 

 marked by longitudinal flutings ; the base of the spine is hollowed 

 into a cup or condyle, which fits over the mamelon of the tubercle. 

 Around the condyle is the collar of the spine which projects above 

 the level of the shaft, and serves for the attachment of the muscles 

 which fix it to the test. A transverse section of the spine (Fig. 

 III.) shows that it is made up of a number of solid wedges, radiating 

 from a central axis, and separated by bands of porous tissue. 1 The 

 smallest spines consist only of tiny needles of porous tissue, and 

 agree in structure with the spines of the second type of appendage. 

 The pedicellariae (Fig. IV.) consist of two or three beak-like 



1 The spine is therefore, according to the terminology of Mackintosh (63), poly- 

 cylic and acanthosphenote. 



