XIV] OPHIUROIDEA 343 



oral tube-feet. The interradii between the arms project inwards 

 over the mouth, as the mouth-angles ; these are lined along their 

 edges and at their tips with broad blunt spines called teeth and 

 mouth papillae, so that they form an efficient strainer and prevent 

 coarse particles entering the stomach (Figs. 158 and 159). The 

 calcareous plate at the apex of each mouth-angle which bears these 

 spines is called the torus angularis (5, Fig. 157). Some Brittle- 

 stars belonging to the genus shown in Fig. 158 have been observed to 

 seize worms by coiling their arms around them and then to devour 

 them. 



We saw that in the star-fish the whole surface is covered with a 

 sensitive skin, but that the tube-feet act as sense-organs as well as 

 being locomotor in function. In the Brittle-stars the sole purpose 

 of the tube-feet is to serve as sense-organs ; they are often covered 

 with little warts consisting mainly of sense-cells with their delicate 

 hairs sticking out all round, just like the batteries of cnidoblasts in 

 Hydra, and in all cases there is a special nervous swelling surrounding 

 the base of each tube-foot called the pedal ganglion (7, Fig. 156). 

 As, however, these tube-feet have lost their power of attaching them- 

 selves by a sucking action to objects and hence are of no use for 

 locomotion, the ampullae have disappeared; and as the action of 

 the ampullae is probably the chief cause of the loss of fluid in the 

 tube-feet of the star-fish, in the Brittle-star, where the loss must be 

 very small, the stone-canal is excessively narrow and the madre- 

 porite instead of being a regular sieve has two pores only, rarely 

 more. It is very curious to find that the madreporite is on the 

 under side of the animal ; in the young Brittle-star it is on the edge 

 of the disc, but in each interradius the upper surface grows more 

 rapidly than the ventral and so it is forced round on to the under- 

 side. To the water-vascular ring are attached four large Polian 

 vesicles, the interradius occupied by the stone-canal alone being 

 without one. The tube-feet are the only sense-organs ; this is more 

 exclusively true than is the case with star-fish, for in the Ophiuroidea 

 the rest of the ectoderm, after having given rise to a cuticle, has 

 disappeared, since the solid mail of plates which the animal possesses 

 appears to render it impervious to sensations of contact. 



The organs of sex are very simple ; they are situated in the disc 

 in the interradii and consist, in each interradius, of several .short 

 pouches. These open into ten sacs, called the genital bursae; 

 one pair being placed in each interradius. These sacs are merely 

 i imaginations of the ectoderm which does not here disappear as 



