\'/> M,\\r,,v.f, r,f /OOLOGY. 



the spines, which are sometimes of a very great 

 length, the test Dears curious little appendages, called " pedi- 

 :'ig. 93, F), and originally supposed to be parasitic. 



of these consists of a stem, bearing two or three, some- 

 times four, blades or claws, which snap together and close 

 upon foreign objects like the beak of a bird. Their action 

 appears to be independent of the will of the animal, and their 

 ;nction is not known ; but they may be regarded as 

 peculiarly modified spines. One function performed by the 

 pedicellari?e, in some species at any rate, is the removal of 



uentitious particles of food. Such particles, on being 

 ejected from the vent, are seized by the pedicellariae, passed 

 on from one to another, and ultimately entirely got rid of. 



In almost all recent Urchins, the test also carries, as shown 

 by I, oven, curious stalked appendages, with button-like heads 

 <(>vcrf(\ with cilia. These so-called " sphaeridia " are supposed 

 to be organs of sense probably of taste. 



Locomotion in the Echinoidea is effected by means of a 

 '.menhir system of contractile and retractile tubes, which 

 constitute the "ambulacra! tubes," or "tube-feet," and are 



'ted with the "ambulac.ral system" of aquiferous canals 

 (fig. 95). From the perforated " madreporiform tubercle " on 



rgtft of the genital plates, there proceeds a membranous 

 canal, known as the "stone" or "sand canal " (s\ whereby 

 water is conveyed from the exterior to a circular tube (r) sur- 

 rounding the (f:r;ophavu<,, ;md constituting the centre of the 

 wa NT -vascular or ambulacra! .system. The function of the 

 tiiadi'-poriform tubercle (ni) appears to be that of permitting 

 the ingress of water from the exterior, but of excluding any 

 solid parhf Irs whirh might be injurious; and as its area is 

 much larger than that of the stone-canal, it admits sea-water 

 not only to the ambulacra! vessels, but also to the body-cavity. 

 It .h'.nl'l !> .ifldrd, however, that the admission of water to 

 ill'- I .ody cavity through tin: madH-poric tubercle is denied by 

 Perrier. The "circular canal" (r) surrounding the gullet is 

 itn .ted between the nervous and blood- vascular rings, and 

 k'ivrs off five branches the "radiating canals" which pro- 

 <l i i'li, illy along the "ambulacral areas" in the interior of 



'H di a). In this course they give off numerous short 

 lateral tubes the "tube-feet" which pass through the "am- 

 i-ill K i.H pore " to g mi the exterior of the test, and terminate 

 in lUCtOrial discs. Besides the r.-nli.-iimj'. ambulacra! canals, 

 there are connected with the cin ul.ir canal certain vesicles of 

 unknown functions (//), known as the "Polian vesicles" 

 (ampulla: Poliana). Five Polian vesicles are generally pres- 



