422 The Animal Kingdom 



connect to cephalic and anal lacunae. In some there is a mid-dorsal 

 vessel which connects with the lateral ones. The blood is most often 

 colorless, but may be green, orange, yellow, or red. The red color 

 is shown to be due to hemoglobin which is contained within definite 

 blood corpuscles. 



The nervous system also has some unique features, including in 

 some species a mid-dorsal nerve cord. The eversible proboscis is 

 contained within the proboscis sheath which is just anterior and dorsal 

 to the digestive system. At one end of the sheath is the proboscis pore 

 through which the proboscis everts in a manner similar to the ever- 

 sion of the finger of a rubber glove. The form of the proboscis is 

 highly variable and often it is armed with spines. 



Most of the nemertines are dioecious, that is, with separate sexes. 

 The gonads are simple sacs present between the lateral intestinal 

 pouches. These open directly to the body surface when the germ 

 cells mature. The eggs develop by determinate cleavage into a free- 

 swimming larval stage, the pilidinm. This stage is suppressed in some 

 species and development is direct. 



Certainly these worms represent the most specialized development 

 of the acoelomate type of construction. 



Cerebratulits is one form commonly found along our east coast. 



