OMNIVOROUS ANNELIDS, &c. 



2 59 



BUCCAL POUCH 



earthworm practically eats its way through the ground, and the 

 earth which has passed through its body is from time to time 

 deposited on the surface in the form of "castings", much as in 

 the case of the Lob- Worm (p. 257). By carefully weighing the 

 castings deposited on a known area Darwin came to the conclusion 

 that in many parts of England 10 tons 

 per acre of soil annually pass through the 

 bodies of these creatures. Earth-worms 

 also devour small pieces of vegetable or 

 animal matter which come in their way. 

 There are several points of interest in the 

 structure of the digestive organs (fig. 467). 

 The mouth opens into a small pouch which 

 can be protruded to the exterior to aid in 

 the taking in of food, and this followed by 

 a muscular pharynx which appears to exert 

 a sort of sucking action. Next comes a 

 slender gullet into which open glands 

 secreting particles of carbonate of lime 

 that perhaps help to neutralize organic 

 acids present in the soil, and the posterior 

 end of this dilates into a rounded crop in 

 which food is temporarily stored, and which again communicates 

 with a rounded muscular gizzard, enclosing small stones, much as 

 in a bird. These help to grind up the food, and thus make up for 

 the absence of jaws. The rest of the digestive tube consists of 

 the long thin-walled intestine, in which the process of digestion 

 is completed. A further fact of interest, observed by Darwin, is 

 that a digestive fluid is poured from the mouth upon bits of 

 vegetable matter, the preparation of which, therefore, begins out- 

 side of the body altogether. 



SIPHON-WORMS (GEPHYREA) 



These are jawless marine worms, some of which are om- 

 nivorous sand-swallowers devoid of bristles. A good example 

 is the Common Siphon - Worm (Sipunculus) (see vol. i, pp. 

 433, 434), found burrowing in sand on many parts of the British 

 and other coasts. The front part of the body is narrow, and when 

 fully expanded is seen to end in a horse-shoe-shaped circlet of 

 short tentacles, within which is the mouth. The greater part of 



Fig. 467. Dissection from above of 

 front end of an Earth-Worm 



