PHYLUM ANNELIDA 



275 



found in the mantle of the squid and in arthropods and certain other 

 animals as well as in the annelids. 



The brain and subpharyngeal ganglion govern the nervous system, 

 initiating and controlling bodily activities. If the brain is removed the 

 worm becomes more active than before, and moves about ceaselessly. 

 This indicates that the brain functions in part as an inhibitory center. If 

 the subpharyngeal ganglion is destroyed, all spontaneous activity stops, 

 and the worm moves momentarily only if it is touched. This ganglion 

 originates the impulses responsible for such activity. Separation of in- 

 hibitory and stimulatory centers in the central nervous system is known 

 only in the annelids, arthropods and chordates. 



122. Nereis and Lumbrkus: Digestive System 



The mouth opens into a muscular pharynx which occupies several 

 segments. In Nereis muscles extending from the prostomium to the back 

 of the pharynx can pull it forward, everting it through the mouth (Figs. 

 15.3 and 15.6). Muscles from the gut to the body wall several seg- 

 ments back can pull it in again. In the middle of the nereid pharynx 

 are numerous small denticles and one pair of large jaws. The jaws lie 

 open at the anterior limit of the everted pharynx. To attack prey the 

 pharynx is everted by its muscles and by a constriction of the body until 



Dorsal Relraclor Pharynx- 



bloodvessel-! muscles -tsophagus 



Brain 



'-Proslomlum 



Cord.-* Di6< 



Mouth 

 ^ _/^Circumph aryn^eal 

 commissure 



Ventral bl.v.-^ Nerve Cord-" Digestive pouch Satpharyngcal gauglion. 



NEREIS 



Intestine' 



"Dorsal blood ves, 

 Hearts 



^Esopba^as 



Ventral IMerve. 

 blood ves. cord 



CalcifcrouS 

 glands 



Brain 

 S'xrProstomiuTn 



'^W.^^Mouth 



Circumpbaryngeal 

 CoramiSSure, 



'Digestive Suhphsryn. 

 pouch oan^lion 



LUMBRICUS 



Figure 15.6. Lateral views of Nereis (above) and Lumbricus (below) with the right 

 body wall removed. The digestive, circulatory and nervous systems are shown. 



