THE TEGUMENTARY SYSTEM. 55 



Cyclops, not because it has more distinguishable organs, but 

 because these organs are so modified as to perform a much 

 greater variety of functions, while they are all coordinated 

 toward the maintenance of the animal, by its wellrdeveloped 

 nervous system and sense-organs. But it is impossible to say 

 that, e. g., the Arthropoda, as a whole, are physiologically 

 higher than the Mollusca, inasmuch as the simplest embodi- 

 ments of the common plan of the Arthropoda are less differ- 

 entiated physiologically than the great majority of Mollusks. 

 I may now rapidly indicate the mode in which physiologi- 

 cal differentiation is effected in the different groups of organs 

 of the body among the Metazoa. 



Integumentary Organs. — In the lowest Metazoa, the integ- 

 ument and the ectoderm are identical, but, so soon as a mes- 

 oderm is developed, the layer of the mesoderm wmich is in 

 contact with the octoderm becomes virtually part of the in- 

 tegument, and in all the higher animals is distinguished as 

 the dermis (enderon), while the ectodermal cells constitute 

 the epidermis (eederoti). The connective tissue and muscles 

 of the integument are exclusively developed in the enderon ; 

 while, from the epidermis, all cuticular and cellular exoskele- 

 tal parts, and all the integumentary glands, are developed. 

 The latter are always involutions of the epidermis. The hard 

 protective skeletons in all invertebrate Metazoa, except the 

 Porifera, the Act i n ozoa, the Echinodermata, and the Tuni- 

 cata, are cuticular structures, which may be variously impreg- 

 nated with calcareous salts formed on the outer surface of the 

 epidermic cells. 



In the Porifera, the calcareous or silicious deposit takes 

 place within the ectoderm itself, and probably the same pro- 

 cess occurs, to a greater or less extent, in the Actinozoa. In 

 those Tunicata which possess a test, it appears to be a struct- 

 ure sui generis, consisting of a gelatinous basis excreted by 

 the ectoderm, in which cells detached from the ectoderm 

 divide, multiply, and give rise to a deposit of cellulose. The 

 test may take on the structure of cartilage or even of connec- 

 tive tissue. In the Vertebrata alone do we find hard exo- 

 skeletal parts formed by the cornification and cohesion of epi- 

 dermic cells. 



In the Actinozoa and the Echinodermata. the hard skele- 



7 



ton is, in the main, though perhaps not wholly, the result of 

 calcification of elements of the mesoderm. In some Mollusks 

 portions of the mesoderm are converted into true cartilage, 



