ANNELIDA 237 



mated 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. ChcEtopoda (bristle-footed). — Annelida with metameres usually well 

 marked both externally and internally; with setse developed from the hypodermis. 

 The coelom 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. 



Pig. 107. 



Fig. 107. Amthitrite ornata, from Verrill's "Invertebrate Animals of Vineyard Sound." 



Subclass I. PolychtBta {with numerous bristles). — Marine Chsetopoda with 

 numerous setas 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 budding. 

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

 stock in that it is sexual. Amphitrite is a beautiful worm which represents the 

 attached or tube-forming types. As the result of their habits such forms tend to 

 lose their segmentation and the appendages of the posterior part of the body. The 

 giUs and tentacles accumulate about the head. These and other types grow abun- 

 dantly in the sand and mud of harbors, amid the vegetation of the bottom, and 

 over exposed objects of all sorts from low-water mark to deep water. Their value 



