SECBETORl SACS Ilf TELEOSTEI. 549 



tactile sense, as shown in the development of papillae and 

 filamentous outgrowths * of the integument, here aids and even 

 replaces the olfactory organ to a' great extent. There is probably 

 not the same necessity, therefore, for the epithelium to be kept 

 in a high degree of sensitiveness as is the case with the Halibut, 

 Plaice, and Turbot. Further, it is well known that a large 

 quantity of mucus is secreted from the external surface in the 

 Soles ; and this, entering the nasal organ with the water-currents, 

 may replace definite secretory sacs and effect the cleansing and 

 preservation of the epithelium as efficiently as the needs and 

 mode of life of the animals require. 



When we come to Cynoglossus^ we find the last stage in the 

 structural specialization, apparently the complete return of 

 the nasal " sacs " to the earliest function of water-retaining. 

 The sacs are not secretory, and, further, there are no tactile 

 filaments round the head ; and it may be that the mucus from the 

 skin enters the nasal cavity and acts as a nasal secretion, just as 

 has been suggested for Solea. 



In the divergent specimen of Gynoglossus, the perforation 

 of the roof of the mouth brings into consideration a totally new 

 function; but by the change that has occurred, the raison d'etre 

 of the former function of water-retaining is still as efiiciently 

 fulfilled. The odoriferous particles which the closed sacs induce 

 to pass over the olfactory epithelium will be drawn through the 

 nasal cavity during the process of respiration, by the movements 

 of the mouth and gill-coTers. The respiratory function, however, 

 although it has arisen secondarily, probably becomes the more 

 important. 



The manner by which the Teleostean fishes respire has recently 

 been carefully described by Dahlgren (/. c), who shows what an 

 important role is played by the maxillary and mandibular 

 " breathing-valves " (fig. 4, h.v.). These are well-developed in 

 the divergent specimen of Gynoglossus ; but it is probable that in 

 the mode of life which these animals lead, the circular fold 

 beneath the central sac which receives the internal nares has 

 taken the place of, or at least may act in the same manner as, the 

 breathing-valves. Both would function when the animal's head 

 was free in the water, but when the jaws were buried in the 

 sand, the nasal respiratory canals and this " respiratory-fold " 



* Eaffaele, F.: " Papille e orgaiii di senso cutaiieo nei Pleuronettidi del 

 genere Solea : nota preliminare." Naples, 1886. 



LINN. JOUEN. — ZOOLOGY, VOL. XXVII. 41 



