iS 1'1.AI'F1SHI;S (IlinKKOSO.MA'lA) 



Ins obsfiA.itioiis iiiul coiuluMdiis may lie nucii m soiiic <letail hero.' Schmidt points 

 out that the normal method of tish respiration, in which first the mouth is opened 

 and botti opercula raised, thus enhirging the buccal cavity and allownij; the water 

 to enter, and siibse<|uently the mouth is closed and the water ejected through the 

 Hill-openings, would be a difficult matter for a fish lying on its side on the bottom. 

 Not only would considerable force be recjuired to raise the operculum of the blind 

 side, but the action of the exhalent current of water would tend to lift the body of the 

 tish from the bottom ; further, the danger of clogging the delicate gill-lamella' with 

 particles of sand or mud which might enter the lower branchial cliamber would be a 

 ver\- real one. These factors have led to the adoption of a special respiratory process 

 in the Hetorosumata. and consequently to the evolution of some interesting mechanical 

 adajitations.- 



It appears that, as a general rule, a Flatfish does not open the gill-opening along 

 its whole length as is the case in other fishes, during either the inspiratory or the 

 expiratory pfiase. The apparatus designed to keep the opening closed is divided 

 into three main parts (Fig. 14) : (i) an " upper " (= dorsal) dermal lobe ; (2) a siphon ; 

 and (J) the " sliding valves " of the branchiostegal membranes. The upper dermal 

 lobe forms a projection above the pectoral fin, which by its close application to the 

 smooth hinder wall of the branchial cavity forms a valve, preventing the entrance of 

 water into the buccal cavity when the pressure within the latter is diminished. Just 

 below the pectoral fin the slightly emarginate edge of the operculum and the hinder 

 wall of the branchial cavity form by their apposition a short tube, the " siphon ", 

 which is sometimes further defined by a distinct ridge on the branchial wall. The 

 margin of the siphon on its opercular side is folded, providing for dilation, and a 

 special fold of meml^rane on the inner surface of the posterior edge of the operculum 

 forms a valve which closes the aperture of the siphon when the pressure within the 

 buccal cavity is diminished. Normally in Flatfishes the siphon serves for the ejection 

 of water during expiration. A " tongue " valve may be present below the siphon : 

 this is an outgrowth from the liinder edge of the operculum, and forms the lower 

 margin of the siphon when the latter is open. In some Flatfishes the soft margin 

 of the operculum in the region of the siphon is crenulate or frilled (Fig. 14). Tlie 

 sliding valves are folds of membrane developed along the edges of the branchiostegal 

 membranes and along the branchiostegal rays, which, by their close application to 

 the smooth wall of the branchial cavity, effectually prevent the entrance of any water, 

 at the same time allowing free sliding movements of the branchiostegal membranes. 



In the form of the sliding valves, as well as in the general manner in which the 

 closing of the lower part of the gill-opening is effected, the various genera of Flatfishes 

 exhibit considerable differences, which are dealt with in detail by Schmidt. He 

 recognises three main types of structure in this part of the respiratory apparatus. 

 In the first, characteristic of all the Pleuronectim. and found also in certain genera 

 of Bothidic (Psetidorhombus. BotJiiis, etc.). the branchiostegal membranes of the two 

 sides are united by means of the posterior (seventh and eighth) brancliiostegal rays, 

 wliich are fu.sed at their ends, and the sliding valves are well developed. In the second 

 type, found in Psettodes, in Eucitharus among the Paralichthinas, and probably 

 throughout the Scophthalmini-e. the two branchiostegal membranes are not fused, 

 the lower edges being free and overlapping (F"ig. 15) : the sliding valves are usually 

 developed. In the tliird type, found only in the families Soleidse and tynoglossidee. 

 the membranes are fused, but there is no actual union of the posterior brancliiostegal 

 rays, and there are no sliding valves. 



Finally, it may be noted that the respiratory apparatus of Flatfishes exhibits a 

 certain degree of asymmetry. The same general structural plan is apparent on 

 both sides of the head, but there are differences in detail — a narrower siphon, more 



' I iini indebted to Mr. B. 1'. f\.ii..i f..r ,1 li.uisLition of tins p.iper. 



■ It must 1)C borne in niintl thai S' lnm.lf-. wc.rlc was carried out wIm.IK .hi pieserird spirnu.ns, 

 and his interpretation of the parts pl.ivcd bv the various slrurturcs iinobed in the respir.ilory 



