270 



THE ANIMAL KINGDOM 



-Stomach 

 •Moulh rNepKridium 



Prostomium 



Ciliated tstnds 



Figure 15.2. An example of the class Archiannelida. Dinophilus, a diminutive (0.5 

 to 2.0 mm. long) annelid that lacks external segmentation but has a metanieric arrange- 

 ment of body organs typical of the phylum. Most of the ventral surface is ciliated, and 

 the animal has a superficial resemblance to a flatworm. (After Meyer.) 



present time the class is maintained as a matter of convenience and not 

 because it is thought to have evolutionary significance in the origin of 

 annelids. 



1 1 8. Nereis and Lumbricus: Habitat and Habit 



Several species of the polychaete genus Nereis are called clamworms 

 (Fig. 15.1). The common east coast form, N. virens, is about one foot 

 long and has a metallic green sheen on the body. On the west coast the 

 common species is the somewhat smaller, metallic blue-green or brown 

 N. vexillosa. They live in sand and gravel, constructing mucus-lined, 

 semi-permanent tunnels from which they forage at dusk. Nereids are 

 omnivorous, gobbling down plant and animal debris and whatever ani- 

 mals they can capture. The single pair of large jaws in the eversible 

 pharynx are adapted for capture but not for chewing. Food is swallowed 

 whole. 



The many species of earthworms are difficult to distinguish. The 

 common European earthworm, Lumbricus terrestris (Fig. 15.1), is now 

 common in the United States also, and is the favorite species for study. 

 It remains in its burrow by day, coming out on damp nights when it 

 can be collected easily. It is largely herbivorous, but acts as a scavenger, 

 eating whatever organic debris is available. 



These two representatives of the polychaetes and oligochaetes will 

 be treated comparatively. In their gross appearance they are more similar 

 than most polychaetes and oligochaetes, but in their detailed anatomy 

 each is a good example of its class. 



119. Nereis and Lumbricus: External Morphology 



The prostomium of Nereis (Figs. 15.1 and 15.3) bears a pair of small 

 tactile tentacles and a pair of stout palps. The palps are used for ex- 

 ploratory probing and their tips are very sensitive to touch and chemi- 

 cals. On the dorsal surface of the prostomium are two pairs of black eyes 



