172 



ORGANS OF DIGESTION. 



carbon, may probably be the excrementitous portion of the 

 biliary secretion, eliminated from the system by a distinct 

 organ, and thus made to serve a double use ; it may, in fact, 

 be analogous to that portion of the bile which is carried di- 

 rectly out of the body by a separate canal in the Doris. 



[ 322. In the ANNELIDA the digestive tube passes straight 

 through the body. The mouth is provided with jaws, and the 

 glands of the intestine are in the form of lateral caecal appen- 

 dages. The circulation is carried on by arteries and veins ; 

 their blood is red, and their respiratory organs are in the form 

 of branchiae, or internal air sacs. 







Fig. 178. The anatomy of the Hirudo medicinalis. 



[ 323. The Leech (Hirudo medicinalis, fig. 178) has a trian- 

 gular-shaped mouth (a), armed with three small teeth, a pharynx, 

 composed of numerous muscles (c) ; the action of which is seen 

 when the animal is engaged in sucking ; the pharynx opens 

 into a very large capacious sacculated stomach, with mem- 

 branous parietes, united by small folds to the enveloping elastic 

 tunic. The stomach is divided into numerous separate cham- 

 bers (/>/,//,/), by transverse processes of the lining mem- 

 brane, communicating with each other by central oval open- 

 ings ; it extends through about three parts of the entire length 

 of the body, where it enters the intestine (m) by a valvular 

 funnel-shaped opening ; this tube passes between the two pos- 

 terior caecal appendages of the stomach, and terminates in a 

 small aperture (n), at the margin of the posterior disc. The 

 gangliated nervous chain (g) is uniform in its development 

 throughout the body, giving off nerves at each ring ; the 

 respiratory vesicles (A) and the lateral vessels (') encircle 

 the body ; the caeca of the digestive tube are seen at q ; the 



