INTEGUMENT 



51 



Fig. 26.— 

 Ciliated cells of 

 the epidermis 



secreted by the underlying epidermal cells, varying in different 

 animals in its thickness, texture, and composition. It is pierced 

 by openings of small size through which secretions of glands are 

 discharged. Other openings are occupied by sense organs of 

 touch, taste or smell connected with nerve endings 

 coming in from below. 



The cuticle of the earthworm is a very thin but 

 tough layer which, if stripped off and examined 

 under the microscope, shows a crisscross arrange- 

 ment of fibrils which give it the appearance of a 

 piece of thin-woven cloth (Fig. 27). Numerous 

 small openings provide a passage for secretions of of the turbel- 

 unicellular mucus glands. The fibrillar structure of clra^inqJalna. 

 the cuticle may be the result of stresses and strains The darkly 

 set up in the cuticle during its formation. In both fhaTdites^H- 

 the flatworm and the earthworm the integument creted by the 



, , <> ,. epidermis. 



has a respiratory function. 



In the group Arthropoda, which includes animals like the 

 lobster, crayfish, spiders, insects, etc., the cuticular layer develops 

 into a stiff armor. Joints between segments of the body and 

 between segments of appendages such as legs or antennae, are 

 provided by a softer cuticle connecting adjacent segments. The 

 soft cuticle is protected by a telescoping of the hard cuticle of one 

 segment over the end of the adjoining one. In 

 insects the cuticle is composed of an organic sub- 

 stance called chitin which is not only hard and 

 tough but also resistant to the action of acids and 

 alkalis. In the crayfish the cuticle consists of an 

 organic material impregnated with lime salts. 

 Beneath the cuticle are the epidermal cells which 

 secrete it. The cuticle of the arthropods not 

 , only serves as a protective outer covering but 

 of cuticle stripped also acts as an exoskeleton to which the body 

 worm an Car " m uscles are attached. Figure 28 shows a section 

 of the integument of the lobster. The cuticle, 

 above, is laid down in stratified layers. The epidermal cells 

 immediately below this layer are large columnar cells con- 

 taining vertical fibrils. On the left side of the figure, the fibrils 

 of the epithelial cells seem to be continuous with fibrils con- 

 nected with muscle cells below. The muscles are attached to the 



