236 



ZOOLOGY 



swallowing the soil for the organic matter which it contains 

 and passing it through its digestive tract. These castings 

 may often be seen at the mouth of the burrows. Worms thus 

 break up the soil, making it more porous and accessible to air, 

 moisture, bacteria, and the rootlets of plants. Darwin esti- 

 mates that three inches of the subsoil is thus brought to the 

 surface in fifteen years through this agency. Doubtless earth- 

 worms bring to the surface materials that renew the soil fertility, 

 replacing substances taken from the surface soil by plants. 



276. Classification. 



Class I. ChcBtopoda (bristle-footed}. Annulata with metameres usually well 

 marked both externally and internally; with setae developed from the epidermis. 

 The co3lom is usually voluminous and is divided into chambers by transverse dis- 

 sepiments. Closed blood-vascular system. Ventral nerve-chain ordinarily > with 

 a distinct ganglion to each segment. 



FIG. 107. 



^a^jm^ 



PIG. 107. Amphitrite ornata, from Yen-ill's "Invertebrate Animals of Vineyard Sound. ' 



Subclass I. Polychata (with numerous bristles}. Marine Chaetopoda with 

 numerous setae typically borne on elevations of the body wall (parapodia). Head 

 usually well differentiated, bearing eyes, antennae, cirri, etc. Branchiae or gills 

 often present. Sexes separate; the reproductive organs simple, and repeated in 

 many segments. A metamorphosis occurs; the larva is known as a trochosphere. 



Nereis, the "sand worm" of fishermen, is a type of this group. Autolytus is a 

 small worm especially interesting because of its power of reproducing by fission. 

 The bud which is freed from the hinder end of the worm differs from the parent 



