24 



Circular 338, U. S. Department of Agriculture 



/fomra 



Fig. 23. — The six-crowned double- 

 pored tapeworm, Dipijlidium sex- 

 coronatiim. Head. Enlarged. From 

 Hall, 1919 



number of kinds of tapeworms, ranging in size from the small hy- 

 datid tsL-pevrovm., Echinococcus granu- 

 losus (synonym: To^nia ecMnococ- 

 cus), which is less than two-fifths 

 of an inch (1 Cientimeter) long, to the 

 marginate tapeworm. Taenia hy da- 

 tig enu (synonym: T. nuirginata)^ 

 which is from 2.5 to over 16 feet (To 

 centimeters to 5 meters) long, or to 

 the broad fish tapeworm, Dijyhyllo- 

 hotJu^um latum (synonym: Dihoth- 

 riocephalus latus). which may attain 

 a length of almost 30 feet (9 meters) . 

 All the dog tapeworms live in the 

 small intestine of the dog. All of 

 them produce eggs which pass out 

 in the feces and which on being 

 swallowed by a suitable host animal 

 will develop to a larval tapeworm, 

 taking the worm of a bladderworm 

 of some sort. The host of this blad- 

 derworm will be in some cases a do- 

 mesticated animal, such as the sheep, 

 cow, or pig; in some cases a wild 

 animal, such as the hare or rabbit; 

 in some cases a fish; and in other 



cases an insect, such as the dog louse or flea. In any case the bladder- 

 worm will not occur in the lumen of the 



digestive tract, but will occur in the tissues, 



such as the lungs, liver, muscles, mesenteries, 



brain, or intermuscular or subcutaneous con- 

 nective tissue, or in the body cavity. In the 



larger animals, such as sheep, cattle, and 



swine, the bladderworms may be very large, 



as in the case of the hydatid, which may attain 



the size of a child's head and which is often 



the size of an orange ; in insects, such as the 



flea or louse, the bladderworm is very tiny, as 



it must be in a host of this size. 



Probably the most common of the tape- 



Avorms in the dog is the common double-pored 



dog tapeworm, Dijyylidium caninum. This 



tapeworm and a closely related species, the 



six-crowned double-pored tapeworm, Dipylid- 



ium sexcoron<itinn^ have more than tAvo 



circlets of hooks on the head (fig. 23), and 



have a genital pore on each side of each seg- 

 ment (fig. 24) ; the thin-shelled eggs occur in 



Q^g capsules (fig. 25) and are often found in 



these capsules in the feces. The segments may 



be white or pink, and the terminal segments 



break off from time to time and either escape 



in the feces or creep out of the anus; such escaping segments are often 



seen and referred to as rectal worms or pinworms. The commoa 



Fig. 24. — Mature segment 

 of six-t'r()\vu(Ml doubU^ 

 pored tai)o\vorm, DipiiUd- 

 iiim sc:r(()r(i)i'itiiiii. P^u- 

 larged. From Hall, 1919, 

 after von Rfltz, 1900 



