VI DEVELOPMENT OF A BREATHING ORGAN I 53 



resulting in the exposure of ii larger length of veins, i.r. ol' a 

 larger amount of blood, to the simultaneous operation of iVesh 

 air or fresh water. Either {a) the skin itself may have de\elo])etl, 

 at more or less regular intervals, elevations, or folds, wliich gradually 

 took the form of papillae, or else {h) an inward folding, or ' in- 

 vagination,' of the skin, or such a modification of the mantle- 

 fold as is described below (p. 172) may have taken place, 

 resulting in the formation of a cavity ]nore or less surrounded 

 by walls, within which the breathing organs were ultimately 

 developed. S(mietimes a combination of l)oth processes seems to 

 liave occurred, and after a papilliform organ has been produced, 

 an extension or prolongation of the skin has taken place, in 

 order to afford a protection to it. Respiration l)y means of a 

 lung-cavity is certainly subsequent, in point of time, to respiration 

 by means of branchiae. 



The branchiae seeni to have been originally ])aired, and 

 arranged symmetrically on opposite sides of the bod}'. It is not 

 easy to decide whether the multiple form of l)ranchia wliich 

 occurs in Chiion (Fig. 59), or the simple form as in Fi-^x/nrlla 



Fig. ,^0. — Cliifnv squamosns L., Bermuda : 

 A, anus ; Br, braiicliiae ; M, mouth. 



Fir;. (30. — Fissnrella virescen!> Sowb., 

 I\inama, sliowing ]io.sitioii of the 

 lirauchiae : Br, liraiicliiae ; E, E, eyes ; 

 F, loot ; M, mantle ; T, T, tentacles. 



(Fig. 60), is the more primitive. Some authorities hold tliat 

 the multiple l)ranchia has gradually coalesced into the snnple, 

 <»thers that the simple form has grown, l)y serial repetition, 

 into the multiple. There appears to l)e no trace of any inter- 

 mediate forms, and, as a matter of i'act, tlie midti])le inanchia is 



