RESPIRATORY ORGANS 



295 



-l.au 



is presented by the Indian Siluroid Saccobranchus. 1 In this 

 Fish a long caecal diverticulum of the branchial cavity extends 

 backwards on each side from the dorsal region of the first 

 branchial cleft to the tail, and in its course is situated internally 

 to the lateral trunk musculature, and close to the vertebral 

 column (Fig. 172). The walls of the caeca are vascular, but no 

 special respiratory structures are developed within their cavities, 

 which, during life, only contain air. 

 In S. singio the right caecum is 

 supplied with blood by an exten- 

 sion backwards of the dorsal portion 

 of the first afferent branchial artery 

 of that side ; the left, on the con- 

 trary, being supplied by the corre- 

 sponding portion of the fourth 

 afferent artery of the same side. 

 In S. fossilis 2 both air-sacs are sup- 

 plied by the fourth afferent branchial 

 artery. The efferent vessels join the 

 fourth efferent branchial artery, 

 right or left as the case may be. 



With perhaps one or two excep- 

 tions, the accessory respiratory organs 

 of Fishes seem to exist for the pur- 

 pose of enabling their possessors to 

 breathe in air. This is certainly 

 the case with the labyrinthiform 

 organs of Anabas and its allies, and also in such Fishes as 

 Amphipnous, Saccobranchus, and the Ophiocephalidae, and 

 probably in others. Nearly all these Fishes are tropical in 

 geographical distribution, more or less amphibious in their 

 habits, and usually possess a remarkable capacity for sustaining 

 life out of water, under conditions which are promptly fatal to 

 ordinary Fishes. Thus, Anabas scandens may be kept alive for 

 days in earthen pots without water, and when free is able to 

 travel short distances on land, especially in the early morning 

 when the dew is on the ground, while Amphipnous frequents 



FIG. 172. Air-sacs of Saccobranchus 

 singio. a.b, The air-bladder en- 

 closed in its bony capsule ; a.c, 

 right air -sac ; a.s, left air -sac ; 

 c.a, bulbus aortae ; l.a.v, afferent 

 vessel of the left air -sac ; r.a.v, 

 afferent vessel of the right air-sac ; 

 r.e.v, efferent vessel of the right 

 sac. (After Hyrtl, altered by 

 Hubrecht.) 



1 Hyrtl, SB. Akad. Wiss. Wien. xi. 1853, p. 302 ; Day, Linn. Soc. Journ. Zool. 

 xiii. p. 198. 



2 Burne, Journ. Linn. Soc. Zool. xxv. 1894, p. 48. 



