STRUCTURE OF ORGANISMS 465 



and related forms it deposits lime which is built up around the polyp. 

 In flatworms a single-layered epithelium is found which in free-living 

 forms is ciliated and attached to a basement membrane. In the adults of 

 the parasitic flatworms the epithelium secretes a firm cuticula. In 

 nemertines and in echinoderms, except the ophiuroids and crinoids, there 

 is a simple ciliated epithelium. 



In the mollusks the skin is made up of an epidermis composed of 

 epithelial cells. This may secrete a cuticula. Under the epidermis is a 

 connective tissue dermis which produces a limy shell. The shell is con- 

 tinually being extended at the margin as it grows and also thickened by 

 addition from within. Brachiopods and bryozoans are similar to mol- 

 lusks in this respect. 



In the remaining non-chordate phyla the epidermis secretes a cuticula 

 over the surface, which may be thin or thick and either flexible or not. 

 The living cells beneath this cuticula are usually termed the hypodermis 

 (Fig. 350). In the arthropods this cuticula contains a large amount of 

 chitin, is thick, and tends to be quite rigid; in the Crustacea it is still 

 further hardened by the addition of lime. 



Connective tissue 



Fig. 350. — Diagramniatic section of an insect's skin. 



The tegument ary system reaches its greatest development in the 

 chordates, in which the skin^ or cuticle, is made up of two layers, an 

 epidermis which is ectodermal in origin, and a dermis, or corium, which 

 is mesodermal and developed from mesenchyme. In the lower chordates 

 and in the aquatic vertebrates it remains relatively simple. In the fishes 

 scales are developed in the dermis, but in living amphibians they occur 

 only in the form of scattered plates in the skin of the backs of a few 

 exotic toads, and of rings of scales in cecilians. In the terrestrial 

 vertebrates the epidermis becomes many-layered and from it are devel- 

 oped scales, hairs, and feathers; the dermis becomes thicker and also 

 contains many structures, as indicated in Chap. L. 



474. Skeletal System. — The skeleton, strictly speaking, is meso- 

 dermal in origin and develops in mesenchyme. In the vertebrates the 

 skeletal parts produced in the mesenchyme of the dermis make up the 

 exoskeleton, which is therefore tegumentary in relationship. To this are 

 usually added the epidermal hard parts. The endoskeleton is formed in 

 the mesenchyme lying deeper in the body. 



A hard endoskeleton is present in only a small number of invertebrate 

 types, though it is seen in some protozoans, where either a calcareous or 

 siliceous framework may be developed in the animal, and in the sponges, 



