XXVI 



THE VOYAGE OF H.M.S. CHALLEXGER. 



By the expansion of the air-bladder the oesophagus and stomach are pressed out of the 

 mouth and the eyes out of their sockets. Cod-fishers are therefore in the habit of 

 puncturing the air-bladder, in order to keep the fish alive. Every larger collection 

 possesses specimens showing the inverted stomach and oesophagus; and they may be 

 safely assumed to have been captured at some distance from the surface. 



We might expect that the air-bladder of deep-sea fishes would offer special modi- 

 fications ; and, indeed, there is sufficient evidence that in many a special muscular 

 apparatus is developed for the compression or expansion of its contents, to enable the 

 fish to rise into a higher, or descend into a lower stratum. But, unfortunately, in all the 

 specimens examined by me the air-bladder was more or less ruptured and broken up, 

 and destroyed by subsequent decomposition. One point only seems to be clearly 

 made out, viz., that abyssal life has exercised no influence on the development of an air- 

 bladder, that is, deep-sea fishes whose nearest surface-relations possess an air-bladder, 

 are provided with this organ also, and vice versa. In none of the abyssal forms examined 

 by me have I found an open communication between the air-bladder and the oesophagus, 

 not even in those which are referred to the Physostomous division. 



The branches of the muciferous system are dilated in many deep-sea fishes to an 

 extraordinary degree. Sometimes it is only the lateral line which is conspicuously wider 

 than in the allied surface forms (Lepidopus tenuis, many Scorpsenidse), but in mauj- 

 others, as in Berycidse, Macruridse, Ophidiidse, and Scopelidse, the l^ranches on the head 

 are enlarged into wide cavities, the walls of which are supported by high ridges of the 

 superficial bones. Frequently the membranes investing these cavities are very thin and 

 after the death of the fish liable to be destroyed, so as to leave bare the deeply sculptured 

 surface of the skull ; in other fishes the outer membranes collapse in consequence of the 

 shrinking of the mucous contents of the cavities, so that the surface of the head appears 

 to be extremely uneven ; whilst again in others, as in the majority of the genus 

 Macritrus, the integument is sufiiciently tough to maintain the natural contours of the 

 head. The arrangement of the cephalic branches is the same as in Teleosteous fishes 

 generally: the rostral l)ranches bifurcate into the frontal and infraorbital, and pass into 

 the scapulary branch which is the commencement of the lateral line ; and the mandi- 

 bulary and prgeopercular branches are likewise invariably present. All these cavities and 

 canals are filled with an immense quantity of mucus, which, in specimens that have not 

 been too long preserved in spirits, swells by immersion in water, and can be pressed out 

 of the apertures of the canals. These apertures may be wide slits, or more or less open, 

 or minute pores with or without a tubule. The physiological use of this secretion, as, 

 indeed, the function of the whole system, is not known. Whether it be regarded as an 

 excretory or as a sensory organ, it is clear that its extraordinary development in so many 

 deep-sea fishes must stand in relation to some one of the abyssal conditions under which 

 they live ; and it is very probable that some special function or functions are superadded 



