90 Animal Morphology. 



fication is based upon that membrane particularly, as the 

 name indicates, its technical nomenclature being Mam- 

 malia. 



Remove this membrane and we run down immediately to 

 aves, reptilia, and pisces, or vertebrata with a thin covering 

 of fibrous sheath over the heart, but entirely devoid of a 

 proper serous coat. 



It is, then, the adding of membrane to membrane in their 

 ever widely differentiating forms, and the blending of each 

 membrane's special functions, which constitutes the diffe- 

 rence between the lowest porifera and the highest order of 

 mammalia. 



And as we remove one membrane and then another, so do 

 we gradually descend from one grade in the animal kingdom 

 to another. But it is probable, as in the insect tribes, that 

 we have not, as the arrested conditions of high life, the 

 permanent conditions of lower life in some almost unnecessary 

 part ; but that the presence of an entire membrane is pre- 

 sented to us in such an organ as the antenna of a butterfly 

 or the sting of an hymenoptera, so that abortive or frag- 

 mentary representatives in lower life are ever presenting 

 themselves as specific peculiarities in certain families and 

 orders of lower life, especially in that of the insect kingdom. 



We now turn to the more difficult tripartite membranes 

 connected with the distribution of food for alimentation to 

 the several tissues, and the destructive process connected 

 with the removal of effete material. 



The three membranes concerned are the ganglionic, 

 lymphatic, and circulatory systems respectively. 



We will begin with the Circulatory membrane. 



It consists of a serous membrane, elastic tissue, and 

 fibrous membrane, and beaded muscles lying internal to the 

 fibrous structure. 



The internal tubing of the veins and arteries consists of 



