CESTODES 189 



CLASS 3. CESTODES.* (TAPEWORMS.) 



Soft, flat parasitic worms in which the body is made up of two 

 distinct parts, a head or scolex and a strobila. The scolex contains 

 either simple or complex suckers and often hooks, the organs of attach- 

 ment : the strobila is composed of a series of similar segments or proglot- 

 tids, each of which contains a complete set of male and female genital 

 organs. In the simplest cestodes, however, no segmentation of the body 

 occurs and but one set of genital organs is present (Fig. 312). The 

 cestodes are digenetic entoparasites which, with a few exceptions, live in 

 two different hosts; as 'adults they live in the intestine of a vertebrate as 

 final host, and as larvae in the muscles or some other compact tissues of 

 an intermediate host. The latter is some animal which is preyed upon or 

 occasionally eaten by the final host. 



The scolex is without a mouth or organs of special sense. The suckers 

 are mostly 2 or 4 in number; their place is sometimes taken by variously 

 formed sucker-like projections called bothria (Fig. 319). Accessory 

 suckers (Fig. 318) are also sometimes present, and in a few forms the 

 entire scolex is absent or rudimentary and the anterior proglottids are 

 modified to form a so-called pseudoscolex, by means of which the animal 

 attaches itself. Hooks are often present on the scolex to assist the para- 

 site in maintaining its position. In the Tceniidae they are situated on a 

 central elevation called the rostellum, and in the Rhynchobothriidae on 

 four long retractile projections called proboscides. 



Behind the scolex is usually a narrow unsegmented region called the 

 neck, after which come the proglottids or segments, which are derived by 

 a process of terminal budding from the scolex and may number from 

 three to several thousand in number, in the different species. The seg- 

 ments nearest the scolex are the youngest and smallest, those at the oppo- 

 site end of the strobila are the oldest and the largest. In Crossobothrium, 

 however, and probably also in other cestodes, a different and much more 

 complex method of growth has been observed,! new segments budding 

 towards the scolex as well as away from it. The genital organs are usually 



* See "Die Parasiten des Menschen," etc., by R. Leuckart, 1879. "Cestodes," 

 by M. Braun, Bronn's Klassen, etc., Bd. 4, p. 927, 1894 to 1900. "Tapeworms of 

 Poultry," C. W. Stiles, Bull. No. 12, Bureau An. Ind., 1896. "Parasites of Fishes 

 of the Woods Hole Region," by B. Linton, Bull. U. S. Fish. Com., Vol. 19, p. 405, 

 1900. "Die thierischen Parasiten des Menschen," by M. Braun, 1903. "Parasites 

 of Fishes of Beaufort," by B. Linton, Bull, of Bur. of Fish., Vol. 24, p. 321, 1905. 

 "Illustrated Key to the Cestode Parasites of Man," by C. W. Stiles, Bull. 25, 

 Hygienic Lab., Wash., 1906. "Taenoid Cestodes of North American Birds," by B. H. 

 Ransom, Bull. U. S. Nat. Mus., 1909. "Die Susswasserfauna Deutschl.," by M. 

 Liihe, 1910. "Index Catalogue," etc. "Cestoda and Cestodaria," by C. W. Stiles 

 and A. Hassall, Bull. 85, Hyg. Lab., Treas. Dep., 1912. 



t See "The Formation of Proglottids in Crossobothrium laciniatum Linton," by 

 W. C. Curtis, Biol. Bull., Vol. II, p. 202, 1906. 



