woR^fs. 



191 



rohita, which ooiiijiviscs small worms wliicli liavo Iho thin jntural jiortions of their 

 bodies curled over on to the ventral side. 



In ccinneetion with the Rhabdoca'la wi' may refer to a small aberrant gronji of 

 worms, the ^Microstomid^e, although they differ in two respects very strikingly from 

 the true Turbellaria. They are niainly not hermaphrodite, an<l multiply not only by 

 ova but also asexually by spontaneous division, like many annelids (A^ais, Aiifo- 

 hjliis, etc.). 



Sub-Class II. — Trematoda. 



The laru'e class of animals to which we now turn our attention offers some of the 

 most interesting life-histories known. The flukes or Trematoda are all jjarasitic upon 

 other animals, and accomplish during their lives strange migrations and metamorjihoses. 

 According to their stage of development varies their habitat; usually the embryo 

 swims about for a short time in the water; it then becomes a parasite by entering the 

 body of its first host, where it changes its form, and by a singular ])rocess of asexual 

 ]iropagation it becomes the parent of several or many individuals belonging to a second 

 generation. The members of the second generation hi some cases multiply further, 

 and the descendants mature to the fin.il sexual stage, while in other inst.ances they 

 change directly into the .adult form. In those species which pursue the more complex 

 metamorjihoses, the parasite may in successive stages infest as many as three different 

 hosts. In general the adult fluke does not live in a host of the same species as the 

 larval worm. 



We cannot better gather a notion of the characteristics of the trematode worms 

 than by following the history of one .species as a concrete example of the h.abits of the 

 class. For this purpose we choose the liver fluke, Dititomum. hepaticum. The adult 

 worm infests the liver of m.ammals. It i.s an hermaphrodite, and every worm produces 

 several hundreds of thousands of small eggs, which it discharges into the bile ducts. 

 The eggs then pass into the intestines, and out with the droppings of the host, in 

 which they may be found abundantly. The embrj-o is developed within the egg shell, 

 and when mature bursts open the little cap or operculum ; this occurs only when the 

 egg is supplied with moisture. If it falls or is washed into some pool the embryo 

 survives its birth, and immediately begins swimming freely in the water. 

 Its form is an elongated cone. Fig. 168, with rounded apex, and measur- 

 ing 0.1.3 mm. in length. The base of the cone is directed forwards, and 

 in its centre is a short, retractile head papilla. The whole surface is 

 covered by cilia, springing from large cells, which form the external en- 

 velope or so-called ectoderm of the embryo. In the interior are two 

 eyes, and other structures, which we will not pause to describe. The 

 embryo is exceedingly active, swimming about like an infusorian, though 

 more rapidly. Now in England, where this worm has been most suc- 

 cessfully studied, there lives in the ponds and ditches of the fields a 

 snail known to zoologists by the name of Iii/mvcmis trunculatus. When 

 the larval Distomum in the course of its gyrations happens to meet one 

 of these unfortunate snails it attacks it. The worm presses its head- 

 papilla against the surface of the snail, and begins spinning like a top 

 around its own axis, and working its body until the tissues of the snail are forced a]iart, 

 leaving a gap through which the embryo squeezes its way into its host. The embryo 



Fro. ifiR.—Free- 

 swimming em- 

 bryo of Dis- 



