178 ANIMALS 



cellular layer. It is of a firm gelatinous substance, which 

 probably adds to the rigidity of the body. In other Coelenter- 

 ates this layer is much thicker, and especially in jelly fishes it 

 forms the major portion of the mass of the animal. In these 

 cases it may contain cellular elements of various kinds, which 

 have moved into it from the ectoderm and entoderm. 



416. Worms generally have no skeleton, but special tissues 

 are sometimes developed in connection with certain organs. 

 The nervous system of nereis is enclosed by a thick covering 

 of a non-cellular connective tissue. 



417. The chitinous cuticula of the Arthropods forms a 

 highly efficient exoskeleton. To this the muscles and other 

 internal organs are attached through the medium of the epi- 

 dermis and basement membrane. The latter is a thin non- 

 cellular layer secreted by the epidermis on its inner surface. 



418. In the MoUusca, also, the exoskeleton is usually ex- 

 tremely well developed. In this case it consists chiefly of thick 

 layers of limey salts, deposited between the cuticula and the 

 epidermis. This ''shell" is formed only by the epidermis of a 

 special fold of the skin called the mantle. 



419. An exoskeleton is rather exceptional among Vertebrates. 

 The scaly covering of the body of most Fishes is an excellent 

 protective structure, but is rather too flexible to be called an 

 exoskeleton. In the head region, however, the scales are often 

 so intimately united as to form a real supporting shell. In 

 special cases this shell is extended over a large portion of the 

 body, as in the gar pike and trunk fishes. 



420. The shell of the turtles is composed partly of the ex- 

 panded ribs of the endoskeleton and partly of bony plates formed 

 in the skin. The whole of the bony portion is covered by horny 

 plates developed by the epidermis. 



421. The jointed shell of the armadillo is made up of bands of 

 dermal bone, while the covering of the scaly ant-eater is com- 

 posed of large, horny scales. 



