14 PARASITES OF THE DOMESTIC ANIMALS 



underlying tissue and a new cuticle has been formed. While the cuticle 

 is at first thin and soft, later it becomes hard and unyielding, therefore 

 the moltings are necessary for the accommodation of growth and occur 

 periodically as long as this growth continues. Chi tin, to which the 

 firmness of the cuticular exoskeleton is due, is an organic substance in 

 which lime salts may be deposited, as occurs in the Crustacea. The 

 skin is never ciliated, nor do ciliated cells occur in any other organs of 

 the body. 



The musculature (Fig. 9) consists of a large number of separate 

 muscles passing from one segment to another and attached at their 

 extremities to the inner side of the skin, their contraction bringing about 

 movements of the segments of the body and appendages one upon the 

 other. They may be attached by so-called tendons, which consist of 

 invaginations of the cuticle surrounded by a corresponding invagination 

 of the epidermis. The muscle fibers are striated and multinuclear. 



The digestive tract (Fig. 2) passes directly, or with little flexion, 

 through the body, the mouth being at the anterior end and usually 

 ventral, the anus posterior. Accessory organs, as salivary glands and 

 liver, may or may not be present. 



Of the circulatory system (Figs. 2 and 3) the most constant portion is 

 the heart, which is usually tubular and located dorsally. On each side of 

 the organ are openings provided with valves through which the blood 

 passes to be propelled forward. From the large arteries the blood may 

 pass directly iato blood sinuses, or it may course through capillaries and 

 veins, though the vascular system is never entirely closed. The blood is 

 usually a colorless fluid with colorless amoeboid corpuscles. 



In aquatic forms (Crustacea) respiration is by gills, while in the air- 

 breathers it may be by tracheae (Figs. 5 and 6), consisting of tubular 

 ramifications from without to within the body, or by peculiar infolding 

 modifications of the integument functioning as lungs. In some of the 

 lower forms respiratory organs are entirely absent, the function in such 

 cases being diffused over the entire body surface. 



In various spaces within the bodies of Arthropoda are frequently 

 found fat bodies, a connective tissue the cells of which, richly laden with 

 fat, serve as a store of nourishment. The fact that products of tissue 

 metabolism, such as uric acid, have been found in the fat body, leads to 

 the conclusion that it also acts as a place of storage for substances of 

 excretion before their elimination by the excretory organs, which latter 

 greatly vary in the different groups. In insects and arachnids these 

 organs are represented by the Malpighian tubes, long glandular canals 

 which open into the posterior portion of the digestive tract. 



The nervous system consists typically of a ventral chain of gangha 

 connected by a double longitudinal nerve cord. In well-developed seg- 

 ments the ganglia are large, and a pair of ganglia to each segment might 



