38 



BEGINNERS' ZOOLOGY 



would chop starfish to pieces, as this only serves to multiply them. 

 This power simulates multiplication by division in the simplest 

 animals. 



Steps in Advance of Lower Branches. — The starfish and other 

 echinodermata have a more developed nervous system, sensory 

 organs, and digestion, than forms previously studied ; most dis- 

 tinctive of all, they have a body 

 cavity distinct from the food 

 cavity. The digestive glands, 

 reproductive glands, and the 

 fluid which serves imperfectly 

 for blood, are in the body 

 cavity. There is no heart or 

 blood vessels. The motions 

 of the stomach and the bend- 

 ing of the rays give motion to 

 this fluid in the body cavity. 

 It cannot be called blood, 

 but it contains white blood 

 corouscles. 



The starfish when first 

 hatched is an actively swim- 

 ming bilateral animal, but it soon becomes starlike (Fig. 60). The 

 limy plates of the starfish belong neither to the outer nor to the inner 

 layer (endoderm and ectoderm) of the body wall, but to a third 

 or middle layer (mesoderm) ; for echinoderms, like the polyps, 

 belong to the three-layered animals. In this its skeleton differs 

 from the shell of a crawfish, which is formed by the hardening 

 of the skin itself. 



Protective Coloration. — Many starfish are brown or yellow. This 

 makes them inconspicuous on the brown rocks or yellow sand. 

 Brightly coloured species are usually chosen for aquaria. 



The Sea Urchin 



External Features. — What is the shape of the body? What 

 kind of sytmnctry has it? Do you find the oral (or mouth) sur- 

 face? The aboral surface? Where is the body flattened? What 

 is the shape of the spines? What is their use? How are the tube 



Fin. 60. — Young starfish crawling upon 

 their mother. (Challenger Reports.) 



