x RESPIRATORY ORGANS 295 
is presented by the Indian Siluroid Saccobranchus.' 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 7 Bee 
of that side; the left, on the con- “~~ 
trary, being supplied by the corre- .. ae 
sponding portion of the fourth «wa.---- My 
afferent artery of the same side. | 
In S. fossilis? both air-sacs are sup-  vez:---$ 
plied by the fourth afferent branchial 
artery. The efferent vessels join the 
fourth efferent branchial artery, 
right or left as the case may be. 
Fie. 172.—Air-sacs of Saccobranchus 
singto. «d.b, The air-bladder en- 
closed in its bony capsule; «@.c, 
With perhaps one or two excep- 
tions, the accessory respiratory organs 
of Fishes seem to exist for the pur- 
right air-sac; a.s, left air-sac ; 
c.a, bulbus aortae ; /.a.v, afferent 
vessel of the left air-sac; 7.0.2, 
afferent vessel of the right air-sac ; 
7.e.v, efferent vessel of the right 
sac. (After Hyrtl, altered by 
Hubrecht.) 
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 
1 Hyrtl, SB. Akad. Wiss. Wien. xi. 1853, p. 302; Day, Linn. Soc. Journ. Zool. 
xiii. p. 198. 
? Burne, Journ. Linn. Soc. Zool. xxv. 1894, p. 48. 
