46 COMPARATIVE ANATOMY. 



from tlie appendages (Vermes, Crustacea). An increase of the surface, 

 which is brought about in various ways, is the mode in which the 

 further complication of branchiaa takes place ; it is very frequently 

 accompanied by a reduction in the number of separate branchial 

 organs. 



The importance of branchiae to the body calls into existence 

 various kinds of supporting arrangements for these organs, 

 which, in their lowest condition, project freely from the surface of 

 the body. Neighbouring parts of the integument being raised up 

 into covering lamellae, the branchiae become hidden in cavities 

 (branchial cavities), and the same tegumentary folds give rise to 

 afferent and efferent canals for the water, which serves for respira- 

 tion (Mollusca, higher Crustacea). In this way the development of 

 respiratory organs may affect other parts of the integument, the 

 direct relation of which to respiration had been lost for a very 

 long time. 



g) Excretory Organs. 

 § 40. 



Just as the gaseous excretory matters are eliminated from the 

 organism by the respiratory organs, so too are there arrangements 

 for eliminating the solid or fluid matters which have become useless 

 to the organism. The whole surface of the ectoderm performs this 

 function in the lower organisms ; in the higher forms of life, on the 

 contrary, there are special organs, dermal glands, which have this 

 function. Of those general arrangements which function as organs 

 of secretion we have to do here with those special ones only which 

 eliminate the excretory matters, and which are distinguished as 

 excretory organs from those glands which secrete matters 

 which are of use to the organism ; these latter are either indepen- 

 dent, or are united to definite systems of organs, of which they are, 

 in that case, specialised parts. 



The excretory nature of the products of secretion of those 

 secreting organs, which are formed by the ectoderm, is least open to 

 doubt, for the products are removed at once from the organism by 

 the emptying of the gland. 



Of the various kinds of organs which open on the surface of the 

 body one sort attains to general importance. These are the kidney- 

 like excreting organs, which eliminate the nitrogenous excreta 

 from the body. These organs are distinctly derived from dermal 

 glands, notwithstanding that in the Vermes, where they seem to 

 have their most simple form, they penetrate deeply into the body ; 

 nor does the fact that in many cases (Annelida, Mollusca) the organ, 

 which in other points also is much modified, opens into the body- 

 cavity, and so connects it with the surrounding medium, and even 

 serves in many groups (Mollusca) as a means for introducing water, 



