METAMOEPHOSES OF DISTOMA. 



147 



Fig. 73. 



grown " parent-nurses " is longer and wider than in any even of the 

 youngest " nurses." (Compare fig. 72, 4, with fig. 72, 5.) 



(387.) We have thus followed the Distoma to its third stage of 

 ascent, and, as no more stages in the generations of these animals have 

 been detected, are not in a condition to trace the origin of the Distoma 

 further back. Steenstrup, however, en- 

 tertains the not unfounded supposition 

 that the " parent-nurses" are not pro- 

 duced from other similar creatures, but 

 that they proceed originally from ova 

 derived from the full-grown Fluke, a 

 supposition which derives additional 

 importance from observations made 

 upon the development of other Entozoa 

 belonging to the Trematode group. 



(388.) In Monostomum mutabile*, for 

 example, which inhabits several of the 

 cranial cavities lined with mucous 

 membrane in certain water-birds, the 

 young embryo is frequently hatched 

 before or just as the ovum is expelled. 

 The newly-hatched young (fig. 73, 1) 

 are elongate-oval, and furnished at 

 their ' anterior extremity with some 

 short lobes, which the animal is able to 

 protrude and retract; and its whole 

 surface is covered with vibratile cilia, by 

 the aid of which it moves readily in the 

 water. In the anterior part of the 



body are two quadrangular spots, which can scarcely be regarded as 

 anything but eyes. The posterior two-thirds of the trunk are occupied 

 by a slightly transparent whitish body (</), which it might be supposed 

 was one of the viscera, as Siebold thought it to be, if it were not that 

 after a time, and some rather vigorous motions, it becomes detached, 

 ruptures the body of its parent, and presents itself as an animal of 

 entirely different appearance from that in which it lay concealed and 

 was developed (fig. 73, 2, 3). 



(389.) Now, since this enclosed animalcule is constantly present in 

 the ciliated young Monostomum, and as there is always but one animal- 

 cule in each individual, and always in the same situation, there must 

 necessarily be some organic connexion between the two, and one en- 

 tirely different from that which it has been supposed could be explained 

 by styling the one a necessary parasite of the other. If one animal is, 

 organically speaking, necessarily connected with another, so that each 

 can be developed only in or around the other, they must belong to one 



* Vide Wiegmann's Archiv fur Naturgeschichte, 1836. 



1. First stage in the development of 

 Monostomum mutabile, after it has quit- 

 ted the egg and is swimming about at 

 liberty: g, internal embryo of "parent- 

 nurse." 2, 3. The same, after its meta- 

 morphosis from an active form into an 

 inactive sluggish creature, which is not 

 itself a mother, but which nourishes 

 within it a progeny from which, in the 

 third generation, a parent animal pro- 

 ceeds. (After Siebold.) 



