206 METAZOAN PHYLA 



ambulacral grooves are closed by the ingrowth of the ventral plates. 

 Owing to the slenderness of the rays none of the viscera extends into them 

 and they are exceedingly flexible and capable of very rapid movement. 

 A madreporite is found on the ventral surface, but the tube feet have 

 lost their ampullae and sucking discs and protrude from the sides of 

 the rays as tactile structures, and also from the surface of the disc, 

 adjacent to the mouth, where they serve to test the food and pass it into 

 the mouth (Fig. 117). The types known as brittle stars and serpent stars 

 are found under stones on the beach at low tide. When the tide is in 

 they wander more or less about the bottom, having somewhat the same 

 feeding habits as the starfishes, but cannot eat objects of any considerable 

 size. Brittle stars are capable of rapid locomotion, but serpent stars are 

 even more active, the rays writhing about Hke the tails of so many snakes 



5p/'ne i\H[Jl| 



rate ^ooi>^M^m 



Fig. 118. — A sea urchin, Strongylocentrotus drobachiensis (Miiller). From a pre- 

 served specimen. The spines have been stripped from the right half; they and the tube 

 feet show on the left. X ^. 



when the animal is strongly stimulated. The basket stars are character- 

 ized by complexly branched rays ending in tendril-like tips. They are 

 found mostly in water of considerable depth, cHnging to masses of sea- 

 weed. Owing to the slenderness of the rays of ophiuroids, they are more 

 likely to be broken than are those of ordinary starfishes. Autotomy is 

 also more frequent, while regeneration is relatively rapid and complete. 



238. Echinoidea. — Echinoidea, or sea urchins (Fig. 118), are ani- 

 mals which have lost their rays and possess a skeleton made up of rows 

 of plates running from the oral to the aboral surface. These plates are 

 divided into five pairs of ambulacral rows, between which are an equal 

 number of pairs of interamhulacral rows. The ambulacral rows are 

 perforated for the exit of tube feet and correspond to those in the ambu- 

 lacral groove of the starfish, while the interamhulacral rows would corre- 

 spond to the interradial plates of the starfish (Fig. 119). One may 

 conceive of a starfish being transformed into a sea urchin by an increase 

 in the vertical diameter of the body and a shortening of the rays until 

 they disappear. With the disappearance of the rays the ambulacral and 

 the interamhulacral bands run up around the side of the body and ter- 



