38 COMPARATIVE ANATOMY. 



by the outermost layer of the protoplasm of a single cell, in which 

 case we have the first example of a denser stratum of the cell becoming 

 separable as a distinct envelope and covering for the rest of the 

 organism. The ectoderm has the function of an organ of defence 

 when its cells secrete a substance which invests, more or less per- 

 fectly, the surface of the body. This substance may harden and 

 give rise to tests or shells, or form a continuous covering for the 

 body, like the carapace of the Arthropoda. 



When a mesoderm is formed, that part of it which becomes con- 

 nected with the ectoderm takes on, in various ways, the functions 

 of an organ of support. The calcareous deposits in the complicated 

 integument of the Bchinodermata are examples of this. 



The activity of the ectoderm in producing firm organs which 

 protect the body is seen also in the Vertebrata, where numerous 

 and varied parts, which function as investing and protecting organs, 

 are produced by it. 



b) Skeleton. 



§31. 



In proportion as the various protective organs which are formed 

 from the ectoderm increase in size or in strength, and at the same 

 time become connected with internal organs, they attain the function 

 of organs of support also. Such organs we designate as the skeleton. 

 The combination of inorganic substances (chiefly calcareous salts) 

 with an organic base plays an important part here. The supporting 

 function of the integument gives rise to numerous adaptations. 

 The combination of the functions of both protection and support 

 is clearly a lower stage as compared with the formation of internal 

 skeletons, which indicate a higher functional differentiation, and 

 function exclusively as organs of support. Here, too, we meet 

 with the most various conditions. The lowest forms, the first 

 beginning of such internal skeletal organs, are solid deposits in 

 the tissues, the separate pieces of which have no connection with 

 one another. The growth and union of these deposits give rise 

 to skeletal formations, which may be also regarded as excretions. 

 Examples of them are found even in the Ccelenterata. When a 

 definite tissue, the properties of which specially fit it for the 

 function of support, is brought into use, the internal skeleton 

 assumes a higher degree of development. The differentiation of 

 cartilage from the indifferent connective tissue is the first ex- 

 pression of this phenomenon. As low down as the Medusas, 

 among the Vermes and among the Mollusca, the employment of 

 cartilaginous tissue for organs of support is commenced, and in the 

 Vertebrata it attains to greater importance, until it is pushed aside 

 by a second and more perfect skeletal tissue — the osseous. 



