THE TEGUMENTARY SYSTEM. 



Cyclops, not because it has more distiiiLruMial .! <-rgans, but 

 because these organs are so modified as to much 



greater variety of functions, while, they are all -r.rdii. 

 toward the maintenance <>t the animal, 1>V its well <l 

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

 that, e. g., the Arthropod^ as a whol< -, are ph\ H.lo^ically 



higher than the Mollnn>-<i, ina.smueh as tin- simplest emi 

 incuts of .the common plan of the Arthropoda a 

 entiated physiologically than the great majm-it \ of Molliiftk*. 

 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. 



Inteijitinentary Organs. In the lowest - 1 . the integ* 



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

 oderrn is developed, the layer of the mesoderm which 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 (ecderon). The connective tissue and muscles 

 of the integument are exclusively developed in the enderon ; 

 while, from the epidermis, all cuticular and cellular < 

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

 The latter are always involutions of the < pid n- hard 



protective skeletons in all invertebrate Metazou, except the 

 Porifera, the Actinozoa, the Echinodermata, and the 7Wt- 

 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 str 

 ure sui generis, consisting of a gelatinous basis excreted by 

 the ectoderm, in which cells detached from the ectod 

 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 . 

 skeletal parts formed by the cornification and cohesion of 

 dermic cells. 



In the Actinozoa and the AW/' "V, the hard 



ton is, in the main, though perhaps not wholly, the reU 

 calcification of elements of the mesodenn. In some Mollusks 

 portions of the mesoderm are converted into true cartilage, 



