310 PHYSIOLOGICAL DEVELOPMENT. 



the higher and lower parts. While the higher parts retain 

 their original characters, the lower parts excrete hard outer 

 layers yielding support and protection. Various stages of 

 the differentiation might be followed. " In Hydractinia" 

 says Prof. Green, this horny layer " becomes elevated at in- 

 tervals to form numerous rough processes or spines, while 

 over the general surface of the ectoderm its presence is 

 almost imperceptible." In other types, as in Cordylophora, 

 it spreads part way up the animaFs sides, ending indefinitely. 

 In Bimeria it " extends itself so as to enclose the entire body 

 of each polypite, leaving bare only the mouth and tips of the 

 tentacles." While in Campanularia it has become a partially- 

 detached outer cell, into which the creature can retract its 

 exposed parts. 



But it is as needless as it would be wearisome to trace 

 through the several sub-kingdoms the rise of these multiform 

 contrasts, with the view of seeking interpretations of them. 

 It will suffice if we take a few groups of the illustrations 

 furnished by the higher animals. 



§ 292. We may begin with those modifications of surface 

 which subserve respiration. Though we ordinarily think of 

 respiration as the quite special function of a quite special 

 organ, yet originally it is not so. Little-developed animals 

 part with their carbonic acid and absorb oxygen, through the 

 general surface of the body. Even in the lower types of the 

 higher classes, the general surface of the body aids largely in 

 aerating the blood ; and the parts which discharge the greater 

 part of this function are substantially nothing more than 

 slightly altered and extended portions of the skin. 



Such differentiations, marked in various degrees, are to be 

 seen among Mollusca. In the Pteropoda the only modification 

 which appears to facilitate respiration, is the minute vascu- 

 larity of one part of the skin. Higher types possess special 

 skin-developments. The Doris has appendages developed 

 into elaborately-branched forms — small trees of blood-vessels 



