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. 
