386 



ANIMAL RESPIRATION 



horny fibres, and there is a similar arrangement in the Dog- Fish, 

 for the thick partitions (gill-arches) between the gill-pouches are 

 supported by jointed hoops of gristle, which unite with one another 

 below, and answer the same purpose. Special muscles are attached 



to this framework, which act so as to 

 adjust and promote the outward flow of 

 water through the gill-clefts. 



There is good reason for thinking 

 that the number of gill-clefts in a Dog- 

 Fish or Shark is a reduction upon an 

 earlier state of things, and it is interest- 

 ing to note that two existing kinds of 

 Shark i^Hexanchus and Hcptanchus) pos- 

 sess respectively six and seven pairs of 

 such clefts ; besides which, it has recently 

 been discovered that in one species of 

 Dog-Fish there are remains (vestiges) 

 of a sixth pair of clefts behind the last 

 existing pair. It is also to be observed 

 that reduction has taken place not only 

 from behind, but also from in front, for 

 just behind the eye of a Dog-Fish or 

 Shark there is a small round hole, the spiracle, with which the 

 pharynx communicates by means of a narrow passage (spiracular 

 cleft). That this is an old gill -cleft which is being utilized for 

 other purposes is conclusively proved by the fact that it contains 

 some small folds which are undoubtedly the last traces of a dis- 

 appearing gill. These folds are known as the false-gill i^pseitdo- 

 b^^anch). The new use to which this old gill -cleft is being put 

 is the transmission of sound to the essential organs of hearing, 

 which are enclosed in a gristly ear -capsule adjoining the front 

 wall of the cleft. In a Skate {Raia batis) there is a thin place 

 in the wall of this capsule to facilitate the conduction of sound. 

 This is a particularly striking case of "change of function", and 

 it acquires much greater interest when we come to investigate 

 the structure of the organs of hearing in such forms as mammals, 

 birds, reptiles, and amphibians. For outside the ear-capsule of 

 these animals there is a cavity or " drum ", covered externally by 

 a tense membrane, from which sound-waves are carried across 

 the drum by one or more little bones (see vol. i, p. 56) to the 



Fig. 516. — Horizontal section through the 

 Breathing Organs of a Dog-Fish or Shark, 

 showing the gill-pouches. The gills are 

 deeply shaded and the course of the breath- 

 ing water indicated by arrows, t. Tongue; 

 gl, gullet. 



