22O 



ZOOLOGY. 



FIG. 99. 



2. External paired appendages (setae, bristles, etc.) not 

 jointed. 



3. There is usually a well-developed body cavity. 



4. The excretory organs are typic- 

 ally paired nephridial tubules, one pair 

 in each segment, connecting the body 

 cavity with the outside. Certain 

 highly modified pairs of these serve 

 as outlets for the reproductive bodies. 



5. The nervous system consists of 

 (i) a supra-cesophageal ganglion 

 (brain), and (2) a circum-cesopha- 

 geal collar or connective uniting it 

 with (3) a ventral chain of ganglia 

 with a ganglion in each segment. 



6. Locomotion is primarily effected 

 by means of the contractions of the 

 body wall, acting on body fluids in the 

 cavity within. 



FIG. 99. Dero, a fresh-water oligochaetous annelid, 

 in optical (frontal) section. Enlarged 30 times, a, 

 appendages; br., brain; d, dissepiments; i, intestine; 

 m, mouth; nph, nephridium; oe, oesophagus; p, pavil- 

 ion, lined with ciliated entoderm; ph., pharynx; pr., 

 processes from the anal segment; s, 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 seg- 

 ment. The anterior half-segment of s' will produce 

 a region like s 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 differ- 

 entiated to deserve 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 

 "-/>. "*"* number 5 of the new posterior worm. What 

 structures then must be developed from the 

 posterior half of the dividing segment? 



