96 SURVEY OF THE REPRODUCTIVE PROCESS 



not developed till some time after the distoma form has 

 been fully acquired. 



The series of changes is still more complicated in the 

 case of many of the Cestoid worms,* and the study of their 

 development has led to the discovery that the Cystic worms, 

 once supposed to be a distinct order of Entozoa, are merely 

 provisional forms, belonging to the early progress of species 

 of the Cestoid division. The egg of a Tape-w^orm, or other 

 Cestoid Entozoon, gives origin to a minute contractile vesicle, 

 armed with six hooks, by which it is enabled to bore its way 

 into the tissues of animals. Allien it is once established in 

 suitable quarters, the primary cyst buds off what must be 

 regarded as the typical form of the order — the Twnia-head 

 — characterized by its four suckers and apical circlet of 

 hooked teeth. By a continuation of the budding process, 

 the Taenia head may be raised on the extremity of a hollow 

 jointed pedicle forming a flexible neck, but no development 

 of proper reproductive organs takes place, so long as the 

 parasite continues in the same locality. On its being 

 transferred, however, to the alimentary canal of a warm- 

 blooded animal, the original caudal vesicle is cast off, and 

 a series of new segments are budded off from the hinder 

 part of the Taenia head. As the gemmation is continuous, 

 and the segments first generated remain adherent to those 

 of subsequent formation, by whose outgrowth they are 

 pushed off from their point of origin, a long jointed ap- 

 pendage is developed, extending backwards from the head. 

 This new formation constitutes the " body" of the Tape- 

 worm, and it is in its segments that the reproductive organs 

 eventually make their appearance. But the ultimate seg- 

 ments are not all directli/ derived from the head, as a pro- 

 cess of transverse sub-division comes in to supplement the 



* AccordiBg to Van Beneden, direct development, without alternation, 

 occurs only in Caryophylleus, a genus inhabiting the intestinal canal of 

 certain fishes. Annals Nat. Hist., 3d Ser., III., 346. 



