DEVELOPMENT OF A BREATHING ORGAN 



153 



resulting in the exposure of a larger length of veins, i.e, of a 

 larger amount of blood, to the simultaneous operation of fresh 

 air or fresh water. Either (a) the skin itself ma}^ have devel- 

 oped, at more or less regular intervals, elevations, or folds, which, 

 gradually took the form of papillae, or else (h) an inward fold- 

 ing, or 'invagination,' 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 more or less surrounded 

 by walls, within which the breathing organs were ultimately 

 developed. Sometimes a combination of both processes seems 

 to have occurred, and after a papilliform organ has been pro- 

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

 in order to afford a protection to it. Respiration by means of a 

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

 tion by means of branchiae. 



The branchiae seem to have been originally paired, and 

 arranged symmetrically on op|)Osite sides of the body. It is not 

 easy to decide whether the multiple form of branchia which 

 occurs in Chiton (Fig. 59), or the simple form as in Mssurella 



Fig. 59. — Chiton squamosus L., Bermuda : 

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



F-' 



Fig. 60. — FissureUa virescens Sowb., 

 Panama, showing position of tlie 

 branchiae : Br, brancliiae ; E, E, eyes ; 

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



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

 multiple branchia has gradually coalesced into the simple, others 

 that the simple form has grown, by serial repetition, into the 

 multiple. There appears to be no trace of any intermediate 

 forms, and, as a matter of fact, the multiple branchia is found 



