10 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8l 



The mouth of the trochophore (fig. 7 D, Mth) opens into an ecto- 

 dermal stomodeum (Stoiii), which leads into a large endodermal 

 stomach, or mesenteron (Ment), of two parts, the second of which 

 communicates with the exterior through a short ectodermal procto- 



M5b 



Mth 



^M5t 



VMcl 



VNv 



Fig. 7. — Structure and development of an annelid trochophore larva. (From 

 Hatschek, 1888-91, but figures A to F turned in position of adult worm with 

 mouth downward.) 



A, blastula stage with endoderm cells (End) differentiated at posterior pole. 

 B, gastrulation, showing a primitive mesoblast cell (Msb) of one side. C, 

 gastrulation completed, mesenteron {Mcnt) detached from ectoderm at posterior 

 end, its anterior end opening through stomodeum (Stoni) and mouth. D, full- 

 grown trochophore larva of Polygordhis. E, diagrammatic view of the muscle 

 system. F, the nervous system. G, ventral surface of a trochophore. 



a, apical tuft of cilia; An, anus; AP, apical plate; b, preoral band of cilia; 

 BIc, blastocoele ; Bp, blastopore : c, postoral band of cilia ; d, circumanal band of 

 cilia; DMcl, dorsal muscle; DNv, dorsal nerve; End, endoderm; Ment, mesen- 

 teron; Msb, primary mesoblast cells; Msd, mesoderm; Mst, metastomium ; Mth, 

 mouth ; Nph, nephridium ; O, eye spot ; Ppt, periproct ; Proc, proctodeum ; Pst, 

 prostomium ; Stoni, stomodeum ; TI, tentacle ; VMcl, ventral muscle ; ]''Nv. 

 ventral nerve. 



demn {Proc) . In development, the endoderm is formed by invagina- 

 tion at the posterior end of the body (fig. 7A, B, iiwrf), but the 

 blastopore {Bp) soon shifts to the ventral surface (C) and elongates 

 forward. The posterior part of the blastopore is then closed ; the 



