39° 



ANIMAL RESPIRATION 



and in Ascidians the minute organisms and organic particles 

 which constitute the food would be liable to pass through the 

 lateral perforations of the pharynx and so wasted, if there were 

 not some special arrangement to prevent it. But in each of 

 these cases there is present a groove along the upper side, and 

 a similar groove along the lower side, along which food is con- 

 ducted to the gullet, entangled in a 

 sticky fluid which prevents their escape. 

 In the Acorn-headed Worm {Bala- 

 noglossjis) (see vol. i, p. 300) the 

 pharynx is divided by projecting folds 

 into a lower food-conducting section 

 and an upper breathing portion (fig. 

 520). The latter possesses a large 

 number of pairs of gill-pouches, which 

 open to the exterior by rounded pores 

 on the upper side of the body. The 

 walls of these pouches are supported 

 by horny rods closely similar to those 

 which stiffen the side-walls of the 

 Lancelet's pharynx. 



In all these Protochordates — and 

 the remark is true for vertebrate ani- 

 mals generally — the pharynx, originally concerned with the diges- 

 tive function only, has acquired a new use, having been pressed 

 into the service of breathingf. Various devices have been evolved 

 to prevent these two distinct duties from interfering with each 

 other, and we have just seen how this is accomplished in the 

 Lancelet, &c. The use of gill-rakers in Fishes has also been 

 explained (see p. 387), and we shall have occasion to note in 

 the sequel the way in which air-breathing vertebrates attempt, 

 with more or less success, to keep the feeding tract distinct from 

 the breathing tract. 



Fig. 520. — Section through an Acorn- 

 headed Worm \Bala}toglossus) in the region 

 of the pharynx. This is incompletely divided 

 into feeding-tube [/■t.') below and breathing- 

 tube [b.t.) above. The latter communicates 

 by means of gill-pouches [g-P.^ with the ex- 

 terior. The course of the breathing-water 

 indicated by arrows, b.c. Body-cavity; d.b v. 

 and v.b.z'. dorsal and ventral blood-vessels; 

 dji. and v.fi. dorsal and ventral ner\'es. 



