ECHINODERMATA 165 



ing through each arm, and discharge it into the feet by 

 means of which they are extended, then sucked fast to an 

 object, and contracted to pull the animal along. The star- 

 fish has no heart, but a circular blood vessel surrounds the 

 mouth and gives off a branch to each arm. 



The nervous system consists of a ring around the mouth 

 and five radial nerves supplying the arms. This system 



FIG. 191. Sea cucumber (Pentactafrondosa). Photograph natural size. 



is easily seen on the ventral surface by separating the two 

 rows of feet in each ambulacral groove. The male can be 

 distinguished from the female only by a microscopic exami- 

 nation of the reproductive glands, of which there are five 

 pairs lying at the junction of the arms with the body. Each 

 gland with its tip toward the point of the arm may be a 

 half inch long, or if ripe two or three inches long. 



Arbacia of dark color and Strongylocenlrotus of a green- 

 ish hue are common representatives of the sea urchins 

 (Echinoidce) which are characterized by the presence of 



