IV 



I 'EKMESINTESTINA L CA NA L 



205 



oesophagus directly behind the mouth, recalls in a striking manner the 

 arrangement found in the Archiannelida and the Terebdlido;. The 

 ciliated fore -gut itself falls into 2 parts, a straight anterior oeso- 

 phagus, and a short posterior fore-stomach into which 2 lateral salivary 



glands enter. 



In the Chcetognatha the very simple fore-gut which lies in the head 

 is compressed transversely, and provided externally with a muscular 

 layer whose fibres run dorso-ventrally. 



B. The Mid-gut. 



The mid-gut comes from the endodermal mesen- 

 teron of the larva or embryo. The relations of the 

 various divisions of the gut to the embryonic gut are, 

 however, clearly established in only a few cases, so 

 that the demarcation of the mid-gut from the fore- and 

 hind-guts is to a great extent arbitrary. In the epi- 

 thelium of the mid-gut numerous gland cells are uni- 



o o 



versally found. 



The mid-gut of the Neinertina runs through the 

 whole body, generally in a straight line, from the 

 oesophagus to near the anus. It lies under the 

 proboscis. It usually has numerous unbranched lateral 

 sacs or diverticula, which occasionally lie one behind 

 the other pretty regularly, and so cause a segmentation 

 of the gut similar to that in the Tndada. In the 

 Hoplonemertma the gut sends off towards the front an 

 unpaired diverticulum under the oesophagus. 



A musculature peculiar to the mid-gut has not yet 

 been discovered in the Nemertina. 



The mid-gut of Malacobdella possesses no lateral diverticula ; 

 it has instead a serpentine coiusf. 



\.* ' ."7 

 In the Nematoda also the mid-gut has a straight \"> ./y 



course through the body. A special enteric muscula- w 



ture seems everywhere wanting. 



, I7 .,, , ,. , , ' . , , ,, , , . FIG. 135. - in- 



With tew exceptions the mid-gut of the Annulata testinal canal 



also runs in a straight line through the body. It is and genital or- 

 almost everywhere surrounded outwardly by a muscular ? ans of a Nemer - 



. . . . i i T tian, diagramma- 



layer, consisting of circular and longitudinal muscle tic. ?., Mouth ,-,.. 



fibres. anus; d, gut; do 



The mid-gut of the Hirwlinea shows mostly paired, J,^ 8 * 1 

 lateral, segmentally arranged c?eca or diverticula, the glands 

 last pair of which is often very long and extends back- outwards b y late - 

 wards on both sides of the hind-gut. Sometimes only 

 this last pair of diverticula is retained, or the diverticula may be 

 altogether wanting (e.g. Nephelis, Lumbricobdella). 



