PARASITIC FLATWORMS 427 



egg in the form of a spherical ciliated larva (Fig. 731) that for a 

 short time carries on an existence in the outer world. This larva 

 occurs among the fish tapeworms. The ex- 

 ternal layer of large ciliated cells may be 

 regarded as an embryonic membrane within 

 which is a narrow fluid filled chamber contain- 

 ing a smaller spherica mass that in fact is the 

 true cestode larva, known as an onchosphere. 

 This larva derives its name from the presence 

 of three pairs of long slender hooks arranged fig. 731- Diphyiiahothri- 



um latum. Free swimming 



at equal intervals around one pole of the sphere ™bryo. Magnified. (After 



^ ^ ^ bcnaumsland.) 



and provided with special muscles that serve 



to push the hooks out and then away from the center so as to 



open up tissue and force the larva through it. 



In most cestodes the onchosphere, surrounded by two or more 

 membranes characteristic in form in particular groups, is retained 

 within the egg-shell until the mass is brought passively into the 

 alimentary canal of a suitable host. Here the onchosphere is set 

 free by digestion and bores its way out of the alimentary canal 

 into the body cavity or vascular spaces. It may remain there 

 free and undergo further development, or by active or passive 

 migration reach a point where it encysts and remains fixed during 

 the period of growth. During this period it develops to one of 

 the larval forms of the group. These forms differ in different 

 subdivisions of the class Cestoda. Among the lower forms they 

 are small, oval or elongated, spindle-shaped, solid-bodied larvae 

 known as plerocercoids, and in the highest groups of cestodes 

 they become large fluid-filled vesicles known as cysticerci or blad- 

 der-worms. Other types occur among other kinds of cestodes. 



These larval forms almost without exception develop in an inter- 

 mediate host. In some cases the larva wanders out later and 

 achieves actively the infection of the adult or final host, but in 

 most instances it is held in the body of the intermediate host 

 until the latter is eaten. Thereupon it is set free by digestion, 

 migrates to the organ which is its normal seat, and enters upon 

 a period of growth that brings it to the fully matured adult 

 form. 



