THE PHYLA ASCHELMINTHES AND NEMERTEA 



233 



Figure 12.8. A ribbon worm, member of the phylum Nemertea. Modified from Coe, 

 1905. 



organs such as eyes and chemoreceptors are similar in construction. 

 Nemerteans also tliffer irom the turbellarians in several respects: the 

 mouth and anus are separate openings, a proboscis may be everted 

 through a pore just above the mouth, a circulatory system is present, 

 and the reproductive organs are simple. Because nemerteans tend to be 

 flattened and long, they are called ribbon worms (Fig. 12.8). 



Although the circulatory system and separate anus are important 

 characteristics for locating the Nemertea among the other phyla, the 

 eversible proboscis is their most characteristic feature, for nothing quite 

 like it is found elsewhere in the animal kingdom. It consists of a pro- 

 boscis pore (tig. 12.9), vestibule, proboscis, proboscis cavity and pro- 

 boscis sheath. When the muscular sheath constricts it exerts pressure on 

 the iluid in the cavity, forcing the hollow proboscis to turn inside out 

 through the \estibule and pore. The proboscis never everts all the way 

 because its inner end is anchored to the sheath by a band of muscle. This 

 muscle is the proboscis retractor. Its contraction helps pull the proboscis 

 back inside the sheath. The proboscis is usually longer than the body, 

 and lies somewhat folded within the proboscis cavity. When everted, the 

 outer surface is sticky, and it coils tightly around the prey, drawing it 

 to the mouth. 



In its simplest form the circulatory system (Fig. 12.9) consists of two 

 lateral vessels connected anteriorly by an anterior lacuna above the 

 proboscis vestibule, and posteriorly by a posterior lacuna below the pos- 

 terior end of the gut. Each lacuna is an enlarged space. Additions that 

 are found in some nemerteans include a middorsal vessel and numerous 

 circular connections. The longitudinal vessels are contractile, keeping 

 the colorless blood and many corpuscles in constant motion. In some 

 species the corpuscles contain respiratory pigment. 



The nemertean circulatory system lacks capillaries, and while it 

 does not form an intimate association with many of the body tissues it 

 probably aids in the distribution of nutrients. Even in species with 

 respiratory pigment it is doubtful whether the system has much to do 

 with ordinary respiration when oxygen is available in the environment. 

 It is more likely that the blood serves as an oxygen reservoir for use 

 when the worm burrows into anoxic mud. 



