ORIGIN OF INTESTINAL WORMS. 



15 



cf Agamozooids, but to the most widely various periods and stages 

 of their development. We can, therefore, by careful selection and 

 judicious arrangement of these observations, build up a general 

 view of the complicated process of the alternation of generations 

 in the Trematoda in general. 



The so-called Cercaria offer the best exemplification of the 

 alternation of generations as it occurs in the Trematoda. These 

 Cercarice, which swim about with great activity by means 

 of a cylindrical tail, have long been known; but until the 

 discovery of their real origin and signification, were taken, on 

 account of their diminutiveness, for Infusoria. When, at a 

 recent period, their parasitic nature was recognised, it became a 

 matter of much astonishment that the Cercarice were not 

 derived from parents resembling themselves, but that they 

 originated in peculiar animated, worm-shaped sacs, which were 

 found buried amidst the sexual and digestive organs, in various 

 kinds of fresh-water snails and 

 mussels. The form of the sacs 

 that produce the Cercarice is, not- 

 withstanding the simplicity of their 

 organization, very various ; in ac- 

 cordance with the form and kind 

 of Cercaria to be developed within 

 them. Some kinds of Cercaria-sacs 

 have an oral aperture, and a simple 

 blind intestine, but in others this di- 

 gestive apparatus is entirely want- 

 ing. One series of Cercaria-sacs possesses contractile walls, 

 whilst others again are stiff and inflexible. In one parti- 

 cular group, the Cercaria-sacs are simple shut receptacles ; in 

 another, the sacs ramify and anastomose to a great extent. The 

 whole of these multifariously-shaped Cercaria-sacs enclose within 

 the walls of their bodies a cavity which, besides the intestinal 



Fig. 1. 



Fig. 2. Fi>. 3. 



Fig. 1. A cercaria-sac (two lines long) provided with an elongated alimentary canal 

 the agaraozobid of Cercaria ephemera, a. Oral cavity, b. Alimentary canal, c. A 

 developed Cercaria ephemera, d. Sporulae not yet developed into Cercaria. These 

 sacs are found in Planorbis corneus. Fig. 2. A cercaria-sac the agamozobid of Cercaria 

 armata provided with a very short alimentary canal and remarkable for the two short 

 lateral abdominal processes, found in Lymnaeus stngnalis. Fig. 3. A perfectly simple 

 cylindrical cercaria-sac, having no digestive canal. I found it as the agamozobid of 

 Cercaria sayittifera in Helix pomatia. 



