NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS I33 



S- — The annelidan progenitors acquired a more effective body 

 movement by the attachment of the longitudinal somatic muscle fibers 

 at several successive rings on the body wall, and by the accompanying 

 formation of transverse muscular septa at the resulting integumental 

 grooves. The body region of the wormlike animal in this way became 

 differentiated into a small number of consecutive motor units, the 

 primary somites. To regulate the new muscular mechanism of 

 metameric movement, there was developed from the body surface 

 of contact with the substratum a new somatic nervous system in the 

 form of ventral nerve cords with ganglia corresponding with the 

 myotomes. The primary and secondary nervous systems were then 

 unified by a connection of the ventral nerve cords with the brain, 

 and the somatic elements of the primary system disappeared. The 

 ingrowth of the septal muscles cut the parenchymatous mesoblast 

 bands into segmental blocks, and the latter became excavated by 

 cleavage spaces (primitive coelomic cavities) for the accumulation 

 of body fluid containing waste products. Excretory organs, if present 

 at this stage, were simple nephridial tubules extending from the ecto- 

 derm into the haemocoele, where they were associated with the meso- 

 blast cavities. The inner parenchymal cells lining the cavities formed 

 epithelial coelomic sacs, but the outer cells, being still an undifferen- 

 tiated tissue, were converted into muscle fibers and connective tissue. 

 The secondary muscles thus formed reinforced the primary somatic 

 muscles already present, and eventually became the major part of 

 the muscular system. The germ cells remained in a mass of undiffer- 

 entiated tissue near the posterior end of the body, and the gametes 

 were liberated probably through a pore or temporary rupture of the 

 body wall. The primitive segmented worms evolved in this manner 

 from unsegmented progenitors were the ancestors of the annelids. 



^. — To increase the reproductive function, the subterminal genital 

 region of the primitive oligomerous annelids was enlarged by the 

 successive generation of new somites from its undifferentiated tissue. 

 A series of secondary telohlastic somites duplicating the structure 

 of the primary somites was thus interpolated between the primitive 

 body of the worm and a small postgenital terminal cone containing 

 the anus. The multiplying germ cells spread into the haemocoele of 

 the new somites, and groups of them became lodged in the walls of 

 the coelomic sacs. The ripening germ cells were now discharged 

 into the coelomic cavities, which latter thus became gonadial as well 

 as nephric in function. Since the coelomic sacs as yet probably had 

 no permanent openings, the gametes must have been liberated through 

 temporary pores of the body wall, through secondary genital openings 



