150 ANIMAL FORMS 



The digestive system of the starfishes, with its various 

 subdivisions and appendages, is in some respects more com- 

 plicated than in the other classes. This is most strikingly 

 the case with the serpent-stars, where the entire system for 

 disposing of the minute animals and plants on which it 

 feeds consists of a simple sac communicating with the 

 exterior by a single opening the mouth. 



In the sea-cucumbers large quantities of sand are taken 

 into the body, and the minute organisms and organic mat- 

 ter are digested from it. In the sea-urchins the mouth is 

 provided with five teeth, and the food consists of minute 

 bits of seaweeds, which these snip off. Such diets evidently 

 require a comparatively simple digestive apparatus, for in 

 both it consists throughout its whole extent of a tube of 

 equal caliber, in which the various divisions of esophagus, 

 stomach, and intestine are little, if at all, defined. This 

 is usually somewhat longer than the body, and therefore 

 thrown into several loops ; and in the sea-cucumbers its last 

 division is expanded and furnished with more highly mus- 

 cular walls, which aid in respiration. 



142. Development. With but a few exceptions, the eggs 

 of the echinoderms are laid directly in the surrounding 

 water, and for many days the exceedingly minute young 

 are borne great distances in the tidal currents. During 

 this period they show no resemblance to their parents, and 

 only after undergoing remarkable transformations do they 

 assume their permanent features. In every case they have 

 a five-rayed form in early youth, but in several species of 

 starfishes additional arms develop until there may be as 

 many as twenty or thirty. 



