168 CHAS. B. WILSON. 



wall of the oesophagus aloag its mid-line. Sometimes the 

 sheet is divided^ and attached along either side instead of in 

 the mid-line. 



In fig. 14 several mesenchyme cells are seen just fastened 

 in place and putting out processes ; in later development (fig. 

 15) the individual cells have coalesced into abroad triangular 

 sheet of muscle. One side is attached to the oesophagus from 

 the mouth nearly to the stomach ; a second side is attached to 

 the ectoderm from the mouth back to the angle between the 

 lappet and the oral surface ; while the third side^ somewhat 

 concave, hangs free. The bodies and the nuclei of the indi- 

 vidual cells composing this muscle can be seen for a long 

 time as swellings irregularly distributed through the muscle 

 sheath. 



The oesophagus is ordinarily dilated laterally^ so that the 

 contraction of this muscle dilates it autero-posteriorly, and, 

 when followed by a contraction of the muscular walls of the 

 oesophagus itself, aids materially in generating the currents 

 of water which take in the food particles and eject the waste 

 matter. 



The motion thus produced corresponds also to the breathing 

 movements in the adult worm, and it does not seem rash to 

 suppose that the larva also breathes in this way, and that the 

 muscle under discussion is one of the muscles of respiration. 



4. Circumoral Muscle. — This consists of a strong 

 muscular ring which surrounds the mouth, and of muscle 

 bands which extend backward and forward from the ring to 

 the edge of the lappets (fig. 15). The elliptical mouth ex- 

 tends from the centre of one lappet to that of the other, and 

 across the space intervening between the lip and the anterior 

 and posterior angles of the lappets numerous muscle-fibres 

 are developed. At the side are usually found large cells, 

 which send out fibres reaching to the extreme anterior and 

 posterior walls of the pilidium, as well as shorter lateral ones. 

 These cells are well developed^, and their contraction tends to 

 draw the borders of the oral surface in toward the centre. 

 The ring around the mouth is developed into a well-defined 



