PHYLUM COELHELMINTHES (ANNELIDA) 157 



undivided coelom contains a complete digestive system the intes- 

 tine of which extends posteriorly some distance, then coils back on 

 itself to a dorsal anal opening toward the anterior extremity of 

 the body. The extended anterior extremity of the body is pro- 

 vided with tentacles or a lobed tentacular fold within which the 

 mouth is located. The entire anterior region is capable of inver- 

 sion within the body. 



A single pair of nephridia comprise the excretory apparatus 

 and serve as gonoducts. The nervous system originates in the 

 anterior extremity as a dorsal cerebral ganglion which is joined 

 with the ventral longitudinal nerve cord by a pair of lateral 

 branches. The ventral nerve cord bears no ganglia but gives off 

 lateral branches to the body wall and to the internal organs. 



Though the sexes are usually distinct, persistent gonads 

 are not present. The germ cells have their origin in masses or 

 ridges of cells in the lining of the body cavity and either undergo 

 full development in this location or are discharged early into the 

 coelom where development is completed. 



Sipunculus and Phascolosoma are characteristic genera of 

 the Gephyrea. 



Outline of Classification 



Phylum Coelhelminthes. — Coelomate worms; usually segmented. 



I. Class Chaetognathi. — Marine, pelagic; three segments; bristle 

 jaws. Sagitta, Spadella, Krohnia. 



II. Class Chaetopoda. — Numerous segments with setae; ventral chain 

 of ganglia; blood vascular system. 



a. Subclass Polychaeta. — Parapodia; numerous setae; marine; 

 separate sexes; no clitellum; trochophore larva. 



1. Order Errantia. — Free-swimming. Nereis, Aphrodita, Lcpido- 

 notus, Syllis, Autolytus, Halosydna. 



2. Order Sedentaria. — Tube dwellers. Clymenella, Arenicola . 

 Myxicola, Sabellaria, Hydroides, Spirorbis, Chaetopterus, Aj'i phj ijui^r^ 



b. Subclass Oligochaeta. — Clitellum; no parapodia; few setae; not 

 marine. 



1. Order Microdrili. — Small; few segments; aquatic; a.sexual 

 reproduction common. Tubifex, Dero, Nais, Chaetogaster, Aeolo- 

 soma, Pristina, Mesenchytraexis. 



2. Order Megadrili. — Large; many segments; usually terrestrial; 

 sexual. Moniligaster, Perichaeta, Helodrilus, Diplocardia, Lum- 

 bricus. 



III. Class Hirudinea. — Typically thirty-four somites; more annulations 

 than somites; one or two suckers. 



1. Order Rhynchobdellida. — No jaws; introvert at anterior end. 

 Glossiphonia, Placobdella, Piscicola. 



