226 



ZOOLOGY 



5- The nervous system consists 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- 

 p IG 10I tionsof the body wall, acting on body fluids 



in the cavity within. 



7. Development may be either direct 

 or indirect. When indirect, the larva 

 passes through a trochophore stage. 



264. General Survey. The Annul ata 

 though conforming to the type outlined 

 above are very diverse in appearance, 

 habits and internal structure, While the 

 Chaetopoda, 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 Annulata. Metamerism 



FIG. loi. Dero, a. fresh-water oligochaetous annelid, in optical 

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

 d, dissepiments; *, intestine; m, mouth; nph, nephridium; oe\ 

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

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

 front of the anal segment where new segments are continually 

 being formed; z', the zone of fission or budding. This takes 

 place in the middle of a segment. The anterior half-segment 

 of z' will produce a region like z for the anterior half of the 

 worm. The posterior half-segment will produce a head and four 

 segments like those which contain the pharynx (1-4) of the 

 parent worm. 



Questions on the figure. What regions of the 

 digestive tract are sufficiently differentiated to de- 

 serve notice? What is the number of the segment 

 in which fission is taking place? What structures 

 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? 



