70 LIFE-HISTORY OF PARASITES. 



Occasionally, however, this state of quiescence is not absolute ; the 

 parasites move from place to place in a slow and gradual fashion, as 

 might have been expected from the size of the parasites and the 

 tissues that surround them. This is known to occur in certain tape- 

 worms 1 (Tocnia c&nurus, T. serrata, T. marginata) whose embryos 

 develop in the brain or liver of mammals. The bladder-worms, 

 which constitute the second developmental stage of the tape- 

 worms, progress, so long as they remain of small size, in a definite 

 direction by a peristaltic action, and form in this way tunnels 

 and passages, which are subsequently invaded by a growth of 

 connective tissue, and present a striking appearance. Sometimes 

 these passages open into the neighbouring cavities of the body, 

 into which the parasites then fall. This is most generally the case 

 with the tape-worms found in the liver of rabbits and ruminants, 

 which find their way into the body-cavity, where they again 

 become encysted. 



The quiescent stage in the life-history of parasites never takes 

 place in the intestine, but may do so in any other organ of the 

 body, and most generally does so in the connective tissue be- 

 tween the muscles and in the parenchyma of the alimentary 

 canal; some sexually mature parasites are also found in these 

 same organs, and hence the question arises, whether they may 

 not be directly developed from the asexual forms, without any 

 further migration. There are two species in which this certainly 

 does occur; one is Archigetes,* an unsegmented tape- worm of the 

 family Caryophyllseidae (Fig. 47), which is a parasite in the body- 

 cavity of many Xaidae. This worm becomes sexual while yet a 

 bladder-worm, which, in other Cestodes, is only an intermediate 

 stage. Another instance is furnished by the genus Aspidoyaster,' 6 

 which inhabits the pericardial cavity of the fresh-water mussel 

 (Fig. 48), and attains sexual maturity without any further change 

 of habitation. 



All these creatures, however, are parasitic upon invertebrates, 

 a fact of which the importance will appear later on. Among the 

 internal parasites of the Vertebrata we do not know of a single 

 analogous example. We may therefore lay down this general law, 

 that the quiescent stage following upon the wandering embryonic stage 

 does not conclude the life-history of the parasite, which needs rather a 

 radical change in its environment, in other words, a second migration. 



1 See Leuckart, ' ' Blasenbandwiinner, " p. 124. 



2 Leuckart, " Archigetes Sieboldi, eine geschlechtsreife Cestodenamme," Zeitschr. f. 

 tciss. Zool., Bd. xxx. (Suppl.), p. 593, 1878. 



3 Aubert, "Ueber das Wassergefasssystem, u. s. w., d. Aspidogaster conchicola," 

 Zeituchr. f. wiss. Zool., Bd. vi., p, 349, 1855. 



