246 THE FOOD OF ANIMALS 



guarded by a circlet of little sensitive tentacles, which perhaps 

 have some selective power, and at any rate to some extent serve 

 to prevent the entry of undesirable substances, such as animals 

 of too great a size, by bringing about contraction of the body- 

 wall, with closure of the apertures as described above. Such 

 protective measures, however, are only partially effective, for 

 parasitic animals are often found in the pharynx. 



WORM-LIKE PROTOCHORDATES (Hemichorda) 



The only other Protochordate to which allusion need be 

 made here is the Acorn - headed Worm {Balanoglossus) (see 

 vol. i, p. 301). Like the Lancelet, it is a widely -distributed 

 animal, and is found in shallow water imbedded in sand or 

 mud, by swallowing which it obtains sufficient organic matter to 

 serve as food. The pharynx here presents a very interesting 

 specialization, for it is almost completely divided into upper 

 (dorsal) and lower (ventral) sections. Water for breathing pur- 

 poses passes into the former and makes its way to the exterior 

 through gill-slits, while food passes back along the lower section 

 to the intestine. The movement in either case is promoted 

 by the action of the cilia with which the digestive tube is lined. 

 The almost complete separation of breathing and food tracts 

 exemplified by the Acorn - headed Worm is an improvement 

 upon the arrangements found in Lancelet and Ascidians, and 

 reminds one of arrangements serving the same end in higher 

 vertebrates, as, e.g., in human beings, where the air used for 

 breathing enters the nostrils and passes back through the passages 

 of the nose to the pharynx, while the food enters the mouth, also 

 to pass back into the pharynx. But in the Acorn-headed Worm 

 both food and breathing water have to enter the mouth, there 

 being nothing comparable to nostrils. More will be said on 

 this subject when dealing in greater detail with breathing and 

 breathing organs. 



A short account must now be given of some of the leading 

 omnivorous forms among Invertebrates. 



