NO. 3 



INSECT HEAD — SNODC.KASS 



13 



(fig. 9), and by a decrease in the relative size of the cephalic enlarge- 

 ment. The young worm is a cylindrical creature with only a com- 

 paratively small prostomial lobe projecting before the mouth. With 

 the elongation of the body, the alimentary canal and the mesoderm 

 bands are correspondingly lengthened, and the trochophore muscles 

 and nerves are continued into the new region. The external surface 

 of the body of the trochophore is marked ofif into several areas by 

 circular bands of cilia ; the worm body, on the other hand, is con- 

 stricted by transverse grooves into a series of segments, or somites. 

 The segmentation of the adult worm originates in tJie mesoderm 

 bands by the development in the latter of a series of paired coelomic 

 sacs. Secondarily, the mesodermic divisions become impressed u])on 



-Ment 



A 



Fig. 9. — Diagrams of the development of an annelid trochophore larva, and 

 early stage in its metamorphosis into a segmented worm. (F"rom Hatschek, 

 i888-'9i.) 



A, early larval stage, showing a primary mesohlast cell {Msh) of one 

 side. B, later stage in which the mesoblast has formed scattered mesenchyme 

 cells (Msc), and a ventro-lateral band of mesoderm (Msd) in each side of the 

 body. C, early stage of metamorphosis in wliich each mesoderm band has 

 divided into a number of primary segments. 



the body wall, and the segmentation expressed externally by a series 

 of transverse, circular grooves on the intersegmental lines. In the 

 worms, the segments increase in number from before backward by 

 the differentiation of new segments between the last one formed and 

 the periproct. The latter remains as an undifferentiated terminal piece 

 of the body bearing the anus. The prostomial region of the trochophore 

 becomes the prostomium of the adult worm; the metastomial region 

 in the Archiannelida constitutes the first body segment, or that im- 

 mediately behind the mouth ; but in the Polychaeta and Oligochacta 

 the metastomium is said to unite with the next somite to form a com- 

 pound peristomial segment. 



In the adult annelid (fig. 10), the body, as distinguished from the 

 head, is all that part of the worm that lies posterior to the mouth 

 (A, MfJi), and the only differentiated head region is the prostomium 



