2 26 ■ ZOOLOGY 



how is its progress through the digestive tract, and its elimina- 

 tion therefrom efifected? Do you discover any circulation of 

 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. — -Compare specimens of Nereis with the earthworm. 

 Note especially: 



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

 eyes, antennae. 



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



Descriptive Text 



262. The Annelida 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 coelom than 

 in the lower worms. They differ from the Coelenterata 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 peculiar balloon- 

 shaped larva known as the trochophore, being (Fig. 106, E), simi- 

 lar in some respects to the Rotifers. 



263. General Characters. 



1 . Body elongated, bilaterally symmetrical and segmented. 



2. External paired appendages when present not jointed. 



3 . A well-developed coelom present. 



4. One pair of excretory organs (nephridia) in practically all 

 segments, connecting coelom with outside. Certain highly 

 modified nephridia may serve as outlets for the reproductive 

 cells in some species. 



