OR OSSEOUS SYSTEM. 



transverse section of the 

 same spine. In Fig. 9, 1, #, is 

 the round tubercle on which 

 the spine moves ; b represents 

 the muscular fibres which 

 descend from the margin of 

 the spine to the circumfer- 

 ence of the base of the tuber- 

 cle ; c, d, is a distinct part of 

 the base of the spine formed 

 within the capsule of the joint ; and /, f represents the soft 

 secreting fleshy layer which envelopes the whole exterior 

 of the skeleton. The concentric layers by which this spine 

 is enlarged are seen in the transverse section, Fig. 9, 2. These 

 superimposed layers of growth are represented in the lon- 

 gitudinal section of a large spine of echinus mammillatus 

 (Fig. 9, 3,) where a represents the muscular fibres by which 

 it is moved and attached, and b is the compact portion of 

 the spine formed within the capsule of the joint. 



In some of these external spines of echinida the growth 

 appears to be effected by the addition of calcareous matter 

 only to the proximal extremity, or fixed ends of the spines. 

 The lines of growth do not then converge and meet in the 

 longitudinal sections, but diverge and terminate at the sides 

 of the spines. This structure is seen in the spines of the 

 cidaris pistillaris, represented in Fig. 9, 4, 5, 6, where a is the 

 tubercle with a cavity at its summit, as we commonly find 

 in the animals of this genus ; b is the concave base of the 

 spine, with a cavity in its centre, as in the tubercle on which 

 it moves. The compact portion of the spine formed within 

 the capsule is seen occupying the middle of its whole extent, 

 and the successive layers of growth are observed extending 

 from this central portion to the sides along the whole spine. 

 A view of the base of this spine is given at Fig. 9, 5, and a 

 transverse section of its middle is represented at Fig. 9, 6', 

 where we see the concentric layers of growth around the 

 middle compact portion. Interspersed among the spines of 

 the echini we find small, fleshy, cylindrical organs which ter- 

 minate in three moveable calcareous spines. Many of the 

 echinoderma have no external skeleton, and are covered only 

 with a coriaceous irritable skin. 



