ORGANS OF DIGESTION. 343 



are allied to the molluscous classes, which they also resemble 

 in their fixed condition and their testaceous covering, in 

 the adult state. 



IX. Annelida. Notwithstanding the difference in the 

 forms, habits and food of the annelides, there is great 

 similarity in the structure of their digestive organs, which 

 generally pass straight through their elongated body, with 

 the mouth and anus at the opposite ends, with slight gastric 

 dilatations in their course, and with an imperfect develop- 

 ment of the hepatic and salivary glands. This simple 

 condition of the alimentary canal, accords with the animal 

 nature of their food ; but as that food is received in various 

 conditions, sometimes mixed in minute particles with earth 

 or sand, and sometimes consisting of larger animals, there 

 is greater diversity in the masticating organs which, in some 

 of the higher annelides are numerous and complicated in 

 structure, and in others are altogether wanting as in the 

 earth-worms and many of the tubicolous species. The 

 masticating organs generally consist of numerous pairs of 

 lateral superimposed horny unciform maxillae as in higher 

 articulated classes; but in those provided with a sucking 

 organ, as the leech, the mouth is furnished with numerous 

 approximated, hard, sharp, recurved teeth, like those common 

 in the molluscous classes. In some, as the earth-worm, the 

 mouth presents a distinct upper and lower lip, as in the 

 entomoid classes, and in others as the phyllodoce the interior 

 of the mouth is capable of being protruded in form of a large 

 proboscis or like the head of a sipunculus. The wide and 

 capacious mouth of the earth-worm (Fig. 82. D. a.) is 

 furnished with a large upper and a smaller lower lip, soft, 

 fleshy, and of great sensibility, and a small salivary gland, 

 and leads by a narrow oesophageal portion of the canal to a 

 slightly enlarged sacculated stomach, consisting of three 

 continuous cavities, placed immediately behind the genetal 

 organs, about a third from the anterior end of the body. 

 The second of these muscular digestive cavities is lined 

 with a tough coriaceous easily detached coat to protect it 

 from the earthy matter taken in with^the food. The stomach 

 opens into a narrower part of the intestine which continues 

 along the middle of the trunk (82. D. a. b. b.) slightly tor- 

 tuous in its course, and gradually enlarging as it descends 



