VERMES. 63 



Class IV. Annelida (Annulata). Kinged Worms. 



Homonomously segmented worms, with brain, ventral chain of ganglia, 

 closed vascular system, and paired nephridia (segmental organs). Typical 

 larval form the trochosphere. 



Order 1. Archiannelida, small marine worms, devoid of setae and para- 

 podia. Body-wall without a circular layer of muscle. Polygordius 

 (with Loven's larva). Protodrilus. Histriodrilus (Histriobdella). 

 Dinophilus. 



Order 2. Chcetopoda, bristle- worms. Free-living forms, with a more or 

 less prominent praestomium, and paired groups of setaa. Body-cavity 

 entirely enteroccelic ; vascular system of a series of closed tubes. 



Polychseta, marine, with numerous setse imbedded in parapodia. 

 Antennae, palpi, cirri, and gills generally present. Sexes usually separate, 

 and development with a metamorphosis. Nereis. Syllis. Aphrodite, 

 sea mouse. Polynoe. Arenicola piscatorum, lug-worm, Terebella, 

 and Serpula, with tubes. Myzostoma, a small much-modified ecto- 

 parasite, occurring on Crinoids. 



Oligochseta, fresh.- water or terrestrial, with relatively few setse. 

 Devoid of antennae, palpi, cirri, and gills. Hermaphrodites with direct 

 development. Lumbricus terrestris (agricola), earthworm. Lumbri- 

 culus variegatus. Nais proboscidea. Tubifex rivulorum. 



Order 3. Hirudinea (Discophora), leeches. Aquatic hermaphrodite 

 ectoparasites divided into annuli more numerous than the true segments, 

 some of which are fused posteriorly into a terminal sucker, dorsal to 

 which is the anus. Mouth anterior, ventral, and surrounded by a sucker. 

 Setae and parapodia absent. Body-cavity in the form of vascular spaces 

 in the connective tissue which occupies the space between body-wall and 

 gut. Piscicola geometra (on freshwater fish) and Clepsine with pro- 

 trusible proboscis. Hirudo medicinalis (var. officinal is), medicinal 

 leech, with three toothed jaws ; attains sexual maturity in about three 

 years. 



