574 



INVERTEBRATE MORPHOLOGY. 



spines are somewhat enlarged towards the tip, the enlarge- 

 ment containing a poison-gland whose secretion is injected 

 into the wound produced by the spine. Pedicellariae, which 

 have already been noted as occurring in the Asteroidea and 

 the Euryalid Ophiuroidea, are richly developed in the Echi- 

 uoids, more especially in the neighborhood of the mouth and 

 auus. They assume varying forms, in the typical one (Fig. 

 264) being composed of a stalk surmounted by three calca- 

 reous pieces or teeth, hinged upon the 

 stalk, and capable of being divaricated and 

 approximated by means of muscles. Each 

 tooth bears cushionlike elevations which 

 are tactile in function, so that the three 

 teeth are vigorously approximated when 

 touched by any foreign body. In some 

 pedicellariae the teeth are very much re- 

 duced in size, but in their place three 

 mucous glands are developed, structures 

 sometimes found also in association with 

 well-developed teeth. The functions of the 

 pedicellariae may be various ; they may 

 serve for the prehension of prey or for 

 protection, and they have also been seen 

 to remove excreta from the surface of the 

 test in the neighborhood of the anus. In 

 the bilateral Echinoids a third form of spine is found, of 

 small size and covered by a richly-ciliated epidermis. These 

 clavulce, as they are termed, are usually associated together 

 in groups of considerable extent termed Semites or fascioles, 

 occurring especially in the neighborhood of the plates per- 

 forated for the emission of tube-feet (Fig. 263), and in the 

 vicinity of the anus. The clavulse have a rich supply of 

 nerve-fibres, and are on this account supposed to be sen- 

 sory in function, though they may also assist in renewing 

 the water in the vicinity of the tube-feet, which probably 

 assist to a greater or less extent in respiration. A fourth 

 variety of appendage to the test is formed by the sphce- 

 ridia, which consist of a stalk surmounted by an oval 

 mass of carbonate of lime traversed in all directions by deli- 



FiG. 364. — Pbdicel- 



LAIUA FKOM jDorO- 



cidaris papillaia 

 (after Koehler). 

 m — muscle-fibres. 



