ANNULATA 225 



the blood? Direction of flow? Evidences? How accom- 

 plished? Test for ability to receive and respond to stimuli of 

 different sorts. Where are new segments formed? Discover, 

 if possible, instances of fission, by which new individuals are 

 formed. 



260. The Leech. The leech may be studied and compared with the earthworm 

 as to its external features, its habits, mode of locomotion, and the like. If large 

 specimens can be had some members of the class might substitute it for the earth- 

 worm and the results of the studies brought into comparison. 



261. Nereis. If specimens of Nereis can be obtained this worm should be 

 compared with the earthworm. (Even two or three good specimens may be made 

 useful from year to year as comparative demonstration both of external and inter- 

 nal structure.) 



Note especially: 



(a) The specialization of the anterior end; proboscis, mouth, jaws, palps, cirri, 

 eyes, antennae. 



(&) The fleshy supports of the bristles, parapodia. 



DESCRIPTIVE TEXT 



262. The Annulata are separated from the unsegmented 

 worms by the possession of a series of segments or metameres 

 which show on the exterior as rings, and contain similar or 

 homologous organs or similar portions of a continuous organ. 

 There is also a more uniform development of the ccelom than 

 in the lower worms. They differ from the ccelenterata in hav- 

 ing bilateral rather than radial symmetry in the adult condition. 

 The development is often direct, but in many, especially the 

 marine forms, there is a metamorphosis. The larva of these 

 has a peculiar balloon-shaped form, known as the trochophore 

 (Fig. 1 06, ), similar in some respects to the Rotifers. 



263. General Characters. 



1. Body elongated, bilaterally symmetrical and segmented. 



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

 jointed. 



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



4. The excretory organs are typically 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. 

 15 



