206 



COMPARATIVE ANATOMY 



CHAP- 



In the mid-gut of the Oligochceta we distinguish two principal 

 divisions, the oesophagus (which according to recent 

 observations also comes from the endodermal mesen- 

 terori) and the stomach -intestine. The oesophagus is 

 generally a tube with narrow lumen and weak walls. 

 In the terrestrial Oligochceta and the Naidomorpha the 

 oesophagus has somewhere in its course a muscular 

 swelling (seldom double), the muscular stomach 

 (gizzard). Besides this, 3 pairs of glandular lateral 

 sacs enter the posterior portion of the cesophagus of 

 the LumbricidcE and related terrestrial Oligochceta ; these 

 are known as the lime-secreting glands, or Morren's 

 glands. 



The large stomach-intestine often shows constric- 

 tions between th? consecutive segments. In the 

 Limibriciclce the absorbing surface of the stomach in- 

 testine is increased by the fact that its dorsal wall is 

 folded longitudinally into the lumen of the intestine, 

 and so forms a tube lying in the dorsal middle line 

 of the gut, but open longitudinally towards the body 

 cavity; this is the typhlosolis (Fig. 165, p. 250, ty). 



In the mid-gut of the Polychceta also we can dis- 

 tinguish an anterior division, mostly shorter and nar- 

 rower, from the posterior mder stomach-intestine. At 

 the boundary between the two special glands may 

 enter, like the Morren's glands of the Oligochceta. The 

 stomach intestine usually shows successive segmental 

 swellings, caused by constrictions at the partitions be- 

 tween the segments. These swellings are often pro- 

 •erticuia of the longed laterally into caeca ; these are particularly long 

 mid-gut ; d-2, pos- jjj AiyhrocUte and the jseouliar species Sphincter ; in 

 vertiouia." ' Aphrodite the cteca themselves are again branched 

 (hepatic tubes). 

 In the Syllidce and Hesionidce two lateral sacs enter the anterioi' 

 end of the mid-gut (or posterior end of the fore-gut f) ; these can be 

 filled with air like a swim-bladder. The mid-gut usually has a straight 

 course, but in the Chlorhcemidce, Amphictenidce, and above all in Stern- 

 aspis, it forms more or less striking loops. 



The Capitellidce and a few Euniciclce are distinguished by the posses- 

 sion of an aceessopy intestine ventral to the main intestine, and 

 opening into it anteriorly and posteriorly in the Capitellidce, but in the 

 Euniciclce, as it appears, only anteriorly. In Capitella ventral ciliated 

 channels run forwards along the oesophagus and backwards along the 

 hind-gat from these openings. The constituents of food are never met 

 A^ath in this accessory intestine, and it probably has a respiratory function. 

 In the Echiuriclce (Fig. 137) a very striking mid-gut lies in 

 numerous loops and windings around the longitudinal axis of the body. 



Fig. 136.— In- 

 testinal canal 

 of Hirudo medi- 

 cinalis. a, Phar- 

 jTix : &, anus ; di, 

 paired lateral di- 



