300 



EMBRYOLOGY 



Annelida (Salexsky). Thus the oesophagus may be formed of ectoderm 

 (Plleolaria, Liimbricus), but it is maintained that it may also be for the 

 greater part of entodermal origin (Pxi/gmobraiichus, Rhynchelmis). The 

 conditions in Ijopadorhy nchus are peculiar ; here the wide ciliated 

 stomodfEum (the larval fore-gut) is not directly converted into the 

 oesophagus, but constitutes a transitory structure. On the wall of the 

 stomodffium two cushion-like thickenings make their appearance, which 

 become hollow, and form two small sacs (Figs. 139 and 135 as). These 

 become considerably enlarged, surround the stomodffium, and finally 

 grow together, after the stomodaeum has closed and separated from the 

 ectoderm. The detached stomodjeum is now seen as a ciliated sac, 

 surrounded by the likewise saccular oesophagus. This finally unites 

 permanently with the ectoderm and entoderm (Kleinenberg). 



Fig. 139(135).— Sap:ittal section of a larva of Lopadorhynchus (after Kleinenbehg). 

 d, intestine ; mp, muscle-plate ; nji, neural plate ; ces, fundament of the supra-ceso- 

 phageal ganglion; so, apical organ ; st, stomodffium (fore-gut of the larva); lo, pre- 

 oral ciliated band. 



The musculature and peritoneal covering of the intestine 

 are supplied by the splanchnic layer of the mesoderm. The 

 chlorogogenous cells which surround the intestine are con- 

 sidered as excretory organs, and have the same origin and 

 significance as the so-called pericardial glands occurring on 

 the blood-vascular system, and are also outgrowths of the 

 same layer (Grobren). In the Lumbricida? the typhlosole 

 arises in the dorsal median line of the intestine as a more or 

 less deep groove-like infolding of its entire wall. 



