ANNELIDA 



227 



5. The nervous system consisting of (i) a supra-esophageal 

 ganglion (brain), and (2) a circum-esophageal collar or con- 

 nective uniting it with (3) a ventral chain of ganglia with a 

 ganglion in each segment. 



6. Locomotion is primarily effected by means of the contrac- 

 FiG. loi. tions of the body wall, acting on body fluids 



in the cavity within. 



7. Development either direct or in- 

 direct. When indirect, the larva passing 

 through a trochophore stage. 



264. General Survey. — The Annelida 

 though conforming to the type outlined 

 above are very diverse in appearance, 

 habits and internal structure. While the 

 Chastopoda, — the class to which the forms 

 studied in the laboratory belong, — are 

 taken as the type, the leeches, which have 

 no bristles but possess suckers, are un- 

 doubtedly related, as is shown by their 

 development. The Rotifers and other 

 forms are sometimes included among the 

 relatives' of the Annelida. Metamerism 



Fig. ioi. Dero, a fresh^water oligocheetous annelid, in optical 

 (frontal) section. Enlarged 30 times, a, appendages; &r., brain; 

 dt dissepiments; •. intestine; m, mouth; nph, nephridium; oe\ 

 esophagus; p, pavilion, lined with ciliated entoderm; ph.^ pharynx. 

 pr.. processes from the anal segment; 0, zone immediately in 

 front of the anal segment where new segments are continually 

 being formed; z', the zone of budding. This takes place in the 

 middle of a segment. The anterior half-segment of z will pro- 

 duce a region like 2 for the anterior half of the worm. The 

 posterior half-segment will produce a head and four segment<i 

 like those which contain the pharynx (1-4) of the parent worm. 



Questions on the figure. — What regions of the 

 digestive tract are suflSciently differentiated to de- 

 serve notice? What is the number of the segment 

 in which budding is taking place? What structiures 

 must the anterior half of this segment make? The 

 segment behind the dividing segment becomes num- 

 ber 5 of the new posterior worm. What structures 

 then must be developed from the posterior half of the 

 dividing segment? 



