198 



METAZOAN PHYLA 



suckers take hold firmly. The muscles of the tube feet now begin to 

 contract, forcing the water out of them and back into the ampullae. As 

 these tube feet shorten, those in other parts of the body relax their hold 

 and the animal is drawn forward by just the length of the contracting 

 tube feet. Now the tube feet in the other parts of the body again take 

 hold, after which the ampullae of the anterior rays are once more brought 

 into play. Water is forced into the tube feet which are in advance, 

 causing them automatically to release their hold and allowing them to 

 be again extended and attached in a new place. Thus the animal 

 literally pulls itself along a tube foot's length each time a tube foot acts. 

 But since all the tube feet do not act in unison, progression is a steady 

 forward movement. 



227. Internal Organs. — The starfish has a true coelom, which is very 

 large and reaches to every part of the body. The space in the water- 

 vascular system represents a portion of the coelom cut off from the 





'Py/oras 



Py/onc ccfeca^rr 



Tef^-f-arc/e 



£ye Spot 

 Tube 7€»^/ 



Fig. 112. — Section of a starfish along one radius and the opposite interradius. The parts 

 in the median line of the ambulacral groove are not shown. 



rest. The coelom is everywhere lined with a 'peritoneum, or lining mem- 

 brane, and is filled with coelomic fluid. 



The digestive system of the starfish is an alimentary canal (Fig. 112), 

 shortened by the flattening of the body and considerably modified. It 

 is divided into a very short esophagus, a large thin-walled stomach, a 

 pyloric sac, and a very slender rectum ending at the anal opening, which 

 is small and non-functional. From the pyloric sac a tube extends into 

 each ray and divides into two branches. Each of these branches connects 

 laterally with numerous glandular pouches called pyloric, or hepatic, 

 caeca. These caeca nearly fill each ray. Pouches connected with the 

 rectum are known as rectal caeca. The number of these varies in different 

 species. The stomach seems to secrete only mucus, but the pyloric sac, 

 as well as the glands in the pyloric caeca, forms digestive enzymes which 

 act on proteins, fats, and carbohydrates. 



Starfishes are diecious. The ovaries and testes are much branched 

 organs lying at the bases of the rays, one on each side of each interradius. 

 The sex cells are passed out through a number of ducts which open on 

 each interradial plate. 



