NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS 2"] 



which are frequently to be seen in the process of mitosis. Posteriorly 

 the growing zone is sharply defined from the pygidial region, but 

 anteriorly it passes by gradual transition into the more fully differen- 

 tiated region in front. Its ectodermal cells, Lillie says, must be derived 

 from the last transverse row of cells in the ectodermal somatic plate 

 produced from the 2d cell of the embryo. The space between the 

 ectoderm and the endoderm is filled with a mass of mesoderm cells 

 very probably generated from the mesodermal teloblasts. Anteriorly 

 the mesoderm of the growing zone is shut off by a roughly defined 



Fig. II. — Diagrams illustrating the direct primary segmentation of the body 

 of a larval polychaete, and the growth of the worm by successive addition of 

 secondary teloblastic somites generated in the subterminal zone of growth. 



A, larva with unsegmented soma and mesoderm bands. B, larval soma and 

 mesoderm directly segmented. C, D, successive formation, from subterminal 

 zone of growth, of teloblastic somites interpolated between primary larval 

 somites and terminal pygidium. 



AlCnl, alimentary canal; Coel, coelomic cavity; E, eye; /-///, primary larval 

 somites; IV -IX, secondary teloblastic somites; Msd, mesoderm; MsT, meso- 

 dermal teloblast ; Pip, palpus ; PMsd, prostomial mesoderm ; Prst, prostomium ; 

 Pyg, pygidium ; Soma, body region between prostomium and pygidium in which 

 somites are formed ; 77, tentacle ; ZG, zone of growth at end of soma. 



transverse partition from the coelomic cavity of the somite before it. 

 The first evidence of new somite formation is the appearance of an 

 irregular space in the mesodermal mass of the zone of growth, which 

 enlarges upward around the alimentary canal and becomes the coelomic 

 cavity of the new soinite. (Arenicola has a dorsal mesentery but 

 none beneath the alimentary canal.) The anterior coelomic wall 

 is pressed against the preceding partition and becomes the posterior 

 lamella of the dissepiment thus formed. Longitudinal muscle fibers 

 make their appearance at an early period in the somatic layers of 

 the mesoderm, but the circular muscles, Lillie claims, appear much 

 later, and evidently, as described by Meyer (1901), are derived from 



