84 



ANIMAL BIOLOGY 



is actually bilaterally symmetrical and the radial form is only 

 secondarily assumed — it masks the basic structure. Indeed 

 'Nature's pentagonal experiment*' has produced a series of bizarre 

 forms that have been successful from early geologic time to the 

 present, though they bear little resemblance to other animals. 

 Witness the structure of a common Starfish. (Fig. 46.) 



The body of the Starfish consists of a central disk from which 

 radiate five arms. It is protected by calcareous plates embedded 

 in the tissue, and by short, blunt spines. About the latter are tiny 

 pincers, or pedicellariae, that keep the surface free from debris 



;Tube feet 

 Marginal spines 

 JAdambulacral spines 



Fig. 46. — Starfish, Asterias. A, oral view; B, devouring a Clam. 

 (From Cambridge Natural History.) 



and protect the delicate, protruding dermal rranchiae. The 

 mouth is situated in the disk on the surface that is ventral as the 

 animal crawls along, and the anus on the opposite surface. Near 

 the anus is a small, porous plate, the madreporite, that admits 

 water to a system of tubes, or water-vascular system, that ter- 

 minates along the amrulacral groove on the ventral surface in 

 myriads of ture feet: the unique hydraulic locomotor and food 

 capturing organs. 



The mouth opens into a large stomach that leads into the py- 

 loric sac from which large, glandular pyloric caeca extend into 

 each arm. Above the stomach is a small rectum that passes to the 

 aboral surface, but waste materials are ejected through the mouth. 



