170 PARASITES OF THE DOMESTIC ANIMALS 



individual segments. In the Diphyllobothriidae they are on the flat 

 ventral surface. The number of segments varies from three. or four 

 (Echinococcus granulosus) to several thousand {Diphyllobothrium latum), 

 a fact which gives to some species an enormous size. In the head is a 

 cerebral ganglion from which two principal nerves run backward, 

 usually near the lateral margins of the segments. An excretory, or so- 

 called water-vascular system, extends through the whole length of the 

 worm by two principal trunks which may be connected by vessels 

 running across the posterior margin of each segment, the system ter- 

 minating at the hinder edge of the last. 



Of the Cestoda, two families are to be described as containing species 

 parasitic to domestic animals and man. These are as follows: 



Family I. Tseniidse. 



Family II. Diphyllobothriidae. 



Family I. Tmniibje 



Cestoda (p. 169). — With rare exception, this family includes all of 

 the tapeworms of domestic animals and man in the United States. Its 

 members have the head furnished with four round or oval cup-hke 

 suckers of muscular structure (Fig. 109), which, by their contraction, 

 produce a vacuum affording a close attachment to the intestinal mucosa 

 of the host. These suckers may surround a prominence, — the rostellum 

 (Fig. 95), or in other cases a depression more or less marked. The 

 rostellum may or may not be contractile, and may or may not be armed 

 with hooks. 



As a typical, though not constant, arrangement of the reproductive 

 organs, those of the species Taenia saginata, a tapeworm of man, are here 

 described. Each sexually mature segment (Fig. 90) of this worm has 

 at its margin a protruding genital pore, which, from segment to segment, 

 is irregularly upon alternate sides. This protuberance contains a 

 cloaca-like cavity into which open the vas deferens and vagina, both of 

 which extend laterally to the middle of the segment. Here the vas 

 deferens divides into a number of seminal ducts which are distributed 

 through the supporting tissue and serve to carry the semen from the 

 small spherical testes which are located almost everywhere in the seg- 

 ment. As it approaches the lateral cloacal sac, the duct becomes con- 

 voluted and much distended with the accimiulated seminal fluid: In the 

 vicinity of the cloaca it develops into a cirrus (penis) which' is inclosed 

 in a muscular sheath. 



The vagina bends downward as it passes toward the center of the 

 segment where it unites with the paired wing-like ovaries which are 

 rather large organs consisting of branched tubules. In the posterior 

 and middle portion of the segment is a single organ, Hkewise of branched 



