300 



EMBRYOLOGY 



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

 (Pileolaria, Lumbricus), but it is maintained that it may also be for the 

 greater part of entodermal origin {Psyqmohranchus, Rhynchelmis). The 

 conditions in Lopadorhy 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 

 stomodfeum two cushion-like thickenings make their appearance, which 

 become hollow, and form two small sacs (Figs. 139 and 13-5 as). These 

 become considerably enlarged, surround the stomodasum, and finally 

 gi-ow together, after the stomodseum has closed and separated from the 

 ectoderm. The detached stomodseum 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). — Sagittal section of a larva of Lopadoi'?iimc7iiis (after Kleinenbebg). 

 d, intestine ; mp, muscle-plate ; np, neural plate ; ces, fundament of the supra-oeso- 

 phageal ganglion ; so, apical organ ; st, stomodaeum (fore-gut of the larva) ; w, 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-vascnlar system, and are also outgrowths of the 

 same layer (Grobben). In the Lumbricidee the typhlosole 

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

 less deep groove-like infolding of its entire wall. 



