276 SYSTEMATIC ZOOLOGY. 



the alimentary canal and re-form it, while the starfishes 

 can reproduce lost arms. Hence starfishes with one or 

 more arms much smaller than the rest are comparatively 

 common. 



Reproduction is exclusively by means of eggs, and in 

 the majority the young larva differs greatly from the adult, 

 being bilaterally symmetrical and frequently provided with 

 numerous long arms which may be soft or may be ren- 

 dered rigid by an internal skeleton. The adult echino- 

 derm forms on one side of the larva and gradually in- 

 creases in size, absorbing the flesh of the larva in itself. 



All of the echinoderms are marine, and members of the 

 phylum occur as fossils in the rocks of all ages from the 

 Paleozoic to the present. The echinoderms are divisible 

 into five classes. 



CLASS I. ASTEROIDA (STARFISHES). 



In the starfishes the flattened body is either pentagonal, 

 or has a number of arms, or rays (usually five, sometimes 

 twenty or more), giving it the shape of a star. The 

 mouth is in the centre of the disc which unites the rays, 

 and is always without jaws or other hard parts. In the 

 body-wall are numerous calcareous plates, movable on 

 one another. In the axis of each ray, on the side of the 

 body with the mouth (oral surface), are regularly arranged 

 ambulacral plates, margined on either side by correspond- 

 ing interambulacral plates. In the rest of the surface 

 (aboral surface) no such regularity of plates occurs. The 

 mouth opens directly into a capacious stomach, the extent 

 of which is increased by gastric pouches. The stomach 

 is also partially divided by a constriction into two cham- 

 bers, an oral, cardiac, and an aboral, pyloric, division. 



