Phylum Annelida and Other Wormlike Coelomates 449 



ate gills. Most of these latter forms live in tubes in the mud and ex- 

 tend the gills above the surface of the tube. 



A pair of dorsal ganglia constitute the "brain" of the annelids. 

 From this, nerves pass anteriorly to the sense organs of the head re- 

 gion, and a pair of connectives circle the digestive tract and join 

 ventrally to form a double nerve cord. In each segment, lateral nerves 

 pass to the body wall from ganglia of the ventral nerve cord. Most 

 of the annelids are well supplied with sense organs for perceiving light, 

 chemicals, and touch. 



Both sexual and asexual reproduction are known among the an- 

 nelids. Asexual reproduction usually consists of transverse fission of 

 the body and regeneration of missing parts. Sexual reproduction is 

 extremely varied and generalizations are difificult. Both monoecious 

 and dioecious species are found. Some forms have specialized gonads, 

 others simply bud the gametes from the coelomic walls. A few forms 

 utilize the posterior segments as egg sacs, and have complicated breed- 

 ing cycles and mating habits. In some species, development is direct 

 while in others an unusual larva, the trochophore, develops. 



While most annelids are free-living forms, commensals and para- 

 sites are known. 



The Classes of Annelida. — Within this phylum, four classes are 

 recognized. 



Class I. Archiannelida. Segmentation externally indistinct, with paired ten- 

 tacles on the prostomium, parapodia or setae absent, nervous system connected 

 with the epidermis, usually with a trochophore larva, marine forms. Example : 

 Polygordius. 



Class II. Polychaela. Segmentation distinct, with paired lateral parapodia- 

 bearing setae, tentacles in head region, sexes usually separate, often with trocho- 

 phore larva, mostly marine forms. Examples: Neanthes (formerly Nereis), 

 Chaetopterus, Amphitrite, Glycera, and Eunice. 



Class III. Oligochaeta. Segmentation distinct, with relatively small number 

 of paired setae per segment, monoecious, with clitellum, no larva, development 

 direct, found mainly in moist soil and fresh water. Examples : Lumbricus, En- 

 chytraeus, Nais, Aeolosoma, Tubifex, and Chaetogaster. 



Class IV. Hirudinea. Segmentation distinct, often marked by secondary 

 rings externally, with large posterior sucker and often with a smaller anterior 

 one, no tentacles or setae, anus opens dorsally, coelom more or less filled with 

 parenchymatous tissue, monoecious, no larval stage. Found in both fresh and salt 

 water and on land. Examples : Hirudo medicinalis, Placohdella. 



