134 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 97 



into the nephridia, or by the autotomous separation of the genital 

 somites. At this stage the generaHzed annehds had acquired the 

 fundamental characters common to the higher Annelida, the Ony- 

 chophora, and the Arthropoda. 



5. — The increase in the size of the body by the addition of the telo- 

 blastic genital somites created a demand for a still greater efficiency 

 of locomotion, and, according to the nature of the response to this 

 demand, two divergent groups of worms were evolved from the 

 generalized annelids. The members of one group acquired segmental 

 clusters of eversible and retractile chaetae serving as adjuncts to the 

 somatic muscular system by maintaining a hold on surfaces of con- 

 tact; the members of the other group developed segmental pairs of 

 lobelike outgrowths of the body wall containing extensions of the 

 somatic muscles, which served as primitive legs. The chaetae-bearing 

 forms gave rise to the Chaetopoda ; the lobopod forms were the 

 ancestors of the walking Onychophora and Arthropoda. 



6. — From the primitive chaetopods were evolved the several groups 

 of modern annelids. By the extension of open tubes from the pos- 

 terior walls of the coelomic cavities to the exterior, a more efficient 

 type of excretory organ (metanephridium) was developed, which 

 could serve also for the liberation of the gametes. The Polychaeta 

 are distinguished particularly by the elaboration of external structures 

 of various kinds, while the Oligochaeta and Hirudinea have achieved 

 a higher development of internal organs and functions. The loco- 

 motor powers of the Polychaeta were increased by the development of 

 lateral lobes of the body wall supporting the segmental groups of 

 chaetae, and in most forms each lateral pair of chaetigerous lobes 

 eventually combined to produce a single locomotor organ, the para- 

 podium. The parapodia served for progression on solid surfaces, 

 and became also effective adjuncts to the undulatory motion of the 

 body during swimming. 



y. — The lobopod annelids became further differentiated from their 

 chaetopod relatives by a chitinization of the entire cuticula, and by 

 the suppression of all the cephalic tentacles except one pair probably 

 corresponding with the palpi of the Polychaeta. They also acquired 

 outlets from the coelomic sacs to the exterior, but the exit ducts were 

 formed as diverticula from the ventral walls of the sacs and opened 

 each on the segment of its sac mesad of the base of the corresponding 

 leg. The germ cells were located in the walls of the dorsal parts of 

 the coelomic sacs, and the primitive coelomoducts discharged both 

 excretory matter and the gametes. The coelomic sacs, however, soon 

 became divided into dorsal gonadial compartments and ventral nephric 



