648 - Heredity and Evolution 



Fig. 32-26. One of the sandworms (Neanlbes). 

 Phylum Annelida; Class Polychaeta. Note the un- 

 jointed appendages (parapodia) and the numerous 

 bristles (setae). Left specimen, ventral aspect; right 

 specimen, dorsal aspect. (Copyright, General Biologi- 

 cal Supply House, Inc.) 



lion in the vertebrates, although very clear in 

 (he embryo, may be somewhat obscured in 

 the adult; and there is some tendency for the 

 segments to become fused and modified even 

 among Arthropoda. 



Other Annelid Features. The annelids were 

 among the first animals to develop a well- 

 defined closed circulatory system (p. 319). In 

 such systems, the blood flows through a con- 

 tinuous system of capillaries and larger ves- 

 sels. The blood does not enter the tissue 

 spaces or come into direct contact with the 

 tissue cells. This efficient type of system, 

 which is well adapted to the nutritive, re- 

 spiratory, and excretory requirements of 

 larger animals, reaches an apex of develop- 

 ment in the vertebrate group, where a cen- 

 tralized pump, or heart, is also present. In 

 the annelids, pumping is achieved by a num- 



ber of pulsating muscular vessels, the aortic 

 loops and dorsal blood vessel (Fig. 17-9). 

 Hemoglobin usually serves as the respiratory 

 pigment (p. 364) in the annelids. But the 

 hemoglobin of the annelid is dissolved di- 

 rectly in the plasma and is not carried by 

 specialized red cells. 



Annelids do not possess jointed append- 

 ages, such as are characteristic of arthropods 

 and vertebrates. Among annelids, in fact, 

 appendages are often absent; or they may be 

 mere slender, chitinous bristles (called setae); 

 or at best, they are short fleshy outgrowths 

 (parapodia), usually bearing bristles (Fig. 

 32-26). 



Anterior 

 sucker 



Posterior 

 suckei 



Fig. 32-27. A leech— one of the Hirudinea. This 

 blood-sucking annelid possesses only 34 segments, al- 

 though the number of external creases (annuli) may 

 be greater. (From Genera/ Zoology, by Miller and 

 Haub, Holt, Rinehart and Winston, Inc.) 



