VERTEBRATES THAT BREATHE IN WATER 389 



what is meant by this expression. A gill is an outgrowth from 

 the body, or it may be a collection of outgrowths, the use of 

 which is to assist breathing- in water. It is, in fact, a means 

 of increasing the surface through which the dissolved oxygen in 

 the surrounding water can pass into the blood, while on the other 

 hand waste carbonic acid from the blood diffuses out into the 

 water. In an ordinary bony fish each comb-like aggregate of 

 filaments may be termed a gill in this sense. 



In the lowest animals (Protochordates) which have any claim 

 to be considered vertebrates there are no gills in the sense just 

 explained, but the perforated walls of the pharynx offer a suffi- 

 ciently large surface to serve the purpose of breathing. In the 



KIDNEY TUBULES 



GILL SLITS OVARIES LIVER ATRIOPORE INTESTINE 



Fig. 519. — Lancelet {Amphioxus lanceolaius) 

 Side view, with internal organs seen by transparency. Semidiagrammatic 



Lancelet [Amphioxus), for example, the pharynx is exceedingly 

 large, and perforated by a great number of oblique clefts (fig. 

 519), and, as each of these is again divided by cross-bars, the 

 result is the establishment of a complex basket-work, possessing 

 a very large surface for exchange of gases between the blood 

 and the surrounding water. By means of horny supporting rods 

 the pharynx is stiffened, and the numerous small openings kept 

 from collapsing. Although the Lancelet is low down in the 

 vertebrate branch, it must not be imagined that this complicated 

 arrangement gives any idea of the breathing organs possessed 

 by the first vertebrates evolved, for the richly -ciliated pharynx 

 is a current-producing organ which is essential to feeding (see 

 p. 244), and it has been elaborated in connection with this 

 important function. 



The Ascidians or Tunicates possess a pharynx with side-walls 

 converted into a basket-work (see vol. i, p. 297), not unlike 

 that found in the Lancelet, and, as in that animal, its duty is 

 to cause water to stream in at the mouth, bringing with it food 

 and the oxygen necessary for breathing. Both in the Lancelet 



