230 ZOOLOGY. 



worm in such a way as to produce a forked or otherwise 

 abnormal result. 



269. Ecology. The leeches are aquatic in habit and 

 many of them live on the blood of higher animals, a kind 

 of temporary parasitism; the Polychaeta are marine, and the 

 Oligochseta are chiefly fresh-water or terrestrial in habit. A 

 few of the latter groups are parasitic. Of the aquatic worms 

 some are actively free-swimming, others crawl in and out 

 among the living and dead matter of the bottom, others bur- 

 row in the sand, or secrete a tubular skeleton into which they 

 may retire. Their chief economic importance is that they serve 

 as food for fish and other food-animals. The earthworm, in 

 forming its underground burrows, eats its way into the earth, 

 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 

 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. 



270. Classification. 



Class I. Chatopoda (bristle-footed}. Annulata with metameres usu- 

 ally well-marked both externally and internally ; with setae developed from 

 the epidermis. The ccelom is usually voluminous and is divided into 

 chambers by transverse dissepiments. Closed blood-vascular system. 

 Ventral nerve-chain ordinarily with a distinct ganglion to each segment. 



Sub-class I. Polychceta {with numerous bristles). Marine Chsetopoda 

 with numerous setae typically borne on elevations of the body wall (para- 

 podia). Head usually well differentiated, bearing eyes, antennas, 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 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 gills and 

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



