LESSONS IN ZOOLOGY. 265 







4to 

 trichina is a parasite belonging to the nematoda. There are 



other worms similar to the nematoda, but they need not be con- 

 sidered here. 



The Annelida are worms whose bodies consist of definite seg- 

 ments. The segments are usually furnished with bristles or setae 

 which serve as organs of locomotion. The digestive system con- 

 sists of a mouth, of ten with a proboscis which may be protruded, 

 sometimes with horny jaws, a gullet, a stomach, intestine and 

 vent. The vascular system is sometimes distinct, and sometimes 

 in communication with the body cavity. When distinct the red- 

 dish blood in the vascular system and the fluid in the body cav- 

 ity both contain amoeboid cells. Branchia or gills are often 

 present as respiratory organs, but frequently respiration is car- 

 ried on through the general surface of the body. There are often 

 organs which seem to serve as kidneys, and there are sometimes 

 hepatic appendages. The nervous system consists of a cerebral 

 ganglion above the gullet and a ventral ganglionic chain. The 

 sense organs are paired eye spots, auditory vesicles, and tactile 

 organs. Sexual reproduction is common, but gemmation and 

 fission often occur. Frequently they develop with metamorpho- 

 sis. Sometimes there is a distinct head with tentacles and bran- 

 chia. They are terrestrial and aquatic animals, mostly marine, 

 living generally on animal food; sometimes they are parasitic. 



The Tubicolse feed on vegetable matter and build tubes of 

 sticks, sand, etc., as a protection for their soft bodies. They 

 have external gills and red blood. 



The Oligochseta include the ordinary earthworm or angle- 

 worm. It has but few locomotive bristles, but has well devel- 

 oped digestive organs. Some of this group are, parasitic, but 

 most of them lead an independent life. 



The Hirudinea include the leeches inhabiting both salt and 

 fresh water. The body is ringed but has no appendages. The 

 common medicinal leech has a sucking-disk at each extremity, and 

 the mouth is armed with jaws strong enough to cut through the 

 human skin. When young it feeds on the blood of insects, then 

 of frogs, and not till maturity is a diet of warm blood necessary. 



