226 PHYLUM ECHINODERMATA. 



The larger spines are attached to their tubercles by an elastic 

 ligament which is inserted into a pit on the tubercle and into 

 another pit in the socket of the spine. The smaller spines are 

 covered by a ciliated epithelium, as are the larger spines in their 

 growing state. The epithelial covering is absent from the 

 full grown larger spines except at their base. The spines are 

 attached by ligamentous and muscular sheaths which pass from 

 the part of the spine round the socket to the smooth part of 

 the shell plate round the tubercle. Around the base of the 

 large spines there is a ring of nervous tissue just beneath the 

 ectoderm. 



The large spines * are used in locomotion. The small spines 

 are protective and are arranged round the large spines, and round 

 the pores of the ocular plates, the anal and genital apertures, etc. 

 They can be bent over the protected object, and are without the 

 nerve ring. 



In the Clypeastroids and Spatangoids the spines are small and seta-like. 

 In Asthenosoma special poison spines have been described by the Sara- 

 sins. They have swollen heads containing the poison gland. It is quite 

 possible that the spines are often poisonous. 



In Centrostephanus longispinus there are about fifteen short spines 

 on the interambulacra near the anus, which are in a continual state of 

 rotation, describing a circle with their tips. 



The term epistroma is applied to calcareous deposits which are found on 

 the plates of the test in some forms. 



Clavulae are minute spines with swollen ends and covered 

 with a ciliated epithelium. They are found in the Spatangoida 

 only and are arranged in definite tracts called fascicles or Semites. 

 The arrangement of these tracts is of importance in classifica- 

 tion. The tube-feet within a fasciolar area always differ in 

 structure from those outside it. 



The principal kinds of fascioles are as follows (Fig. 168). (1) The peri- 

 petalous (p), which encloses the petaloid portions of the ambulacra ; (2) 

 the subanal (sa), which encloses a space on the oral side of the anus ; 



(3) the marginal (m) along the border of the shell parallel to the ambitus ; 



(4) the internal (i) which crosses the petals near the apical region ; (5) 

 the laterals (I), which run one on each side from a point on the peripetalous 

 towards the periproct. They are not all present in the same species. 



* For the physiology of the spines see J. v. Uexkull. Zeit. Biol. (2), 

 21, 1899, p. 73, and ibid. 22, p. 447. 



