106 ANNELIDA 



length of the stomach. These communicate with the gastric 

 plexus and receive the efferent branchial vessels from the 

 first four pairs of gills. 



4. Lateral Gastric Vessels. — One on each side receives 

 blood from the gastric plexus. They are clearly distinguish- 

 able only on the anterior half of the stomach. Each com- 

 municates anteriorly with a heart. 



5. Hearts. — Paired structures, located laterally on each 

 side of anterior end of stomach. Each consists of an auricle 

 and a ventricle. Each auricle is really a swelling at the an- 

 terior end of a lateral gastric vessel. The auricle gives off 

 anteriorly a lateral esophageal vessel as well as discharging 

 into the ventricle. The ventricle sends blood into the ventral 

 blood vessel. 



6. Neural Vessels. — One runs along each side of ventral 

 nerve cord. They arise in capillaries at the anterior end of 

 the body. Each communicates with the ventral vessel in 

 segments two to six, inclusive. 



7. Nephridial Longitudinal . Vessels. — Lie just ventral to 

 nephridiopores, one on each side of the body. Each runs the 

 length of the region where nephridia are found. 



8. Dorsal Longitudinal Vessels. — One on each side run- 

 ning parallel to the nephridial longitudinal but dorsal to the 

 level of the notopodial sacs. 



9. In each setigerous segment there is a ring of blood ves- 

 sels formed by the connectives between the neurals, nephridial 

 longitudinals and dorsal longitudinals. These vessels can 

 best be seen in young, transparent worms. 



Muscular System. — Note the conspicuous bands of longi- 

 tudinal muscles. External to these are the circular muscles 

 which can be seen only if some of the longitudinal layer is 

 teased away carefully. Inside these muscles are the oblique 

 muscles which consist of many bands which pass from the 

 midventral region to the dorsolateral part of the body wall 

 on each side. Associated with each setigerous sac, there are 

 six to ten protractor muscles, which are attached to the dorso- 



