274 Alternation of Generations, etc., [Nov., 



perfect one. Here then, the writer recognized in the first polypi 

 form which becomes fixed, a being which originates from the ova 

 of these gelatinous animals, an individual or product unlike the 

 parent. It has its own individuality, which it retains for a time, 

 till finally a change takes place by which numerous individuals 

 are developed, one above another, on a parent stalk. It is in 

 these last developments that the polypi form is changed back to 

 the medusae. 



The next instance which we shall notice, and where the mode 

 of generation tends to support the doctrine announced, is the Salpee, 

 a singular animal well known to voyagers upon the deep. One 

 of these beings is that of a chain of individuals of forty or fifty in 

 number, linked together at their extremities by appropriate or- 

 gans. This chain moves, as a whole, each individual moving in 

 concert with its fellows. In company with the chain salp^e are 

 found some free individuals, but in most respects like the linked 

 ones. In this instance the generation of salpse are alternately 

 free and linked, or associated; the free salpse bringing forth the 

 associated, and the associated bringing forth the free; and it is 

 consequently considered that the free is more advanced, inasmuch 

 as multiplicity indicates a low grade of organization. The free 

 salpa then, is the mother, who brings forth the chain salpae, a 

 depressed type of organization, but which are designed, in its own 

 individual power, to bring forth an offspring whicTi will return to 

 its parent form. 



To us, and we think to many, a more interesting example of 

 this kind of generation is found in the Trematoda, the fluke or 

 liver worms of sheep and other animals. The details of the 

 changes however, in this order of animals, is drawn from the fluke 

 of the snail of stagnant water, Lymea stagnalis. 



The facts as stated by our author are these; the snail is sur- 

 rounded by thousands of little polypi-like animals, or animals 

 with soft extensible bodies, which at one end have the appearance 

 of a neck, above which their appears a border regularly serrated 

 or armed with small teeth within. These creatures are regarded, 

 or have been regarded as a distinct species, and have received a 

 name indicating the same being known among naturalists as the 

 Cercaria echinata? Within the spines spoken of above, there is 

 an oval orifice. The other extremity is terminated by a tail pro- 

 portionally long, and near its base is a sucking disc, by which 

 the animal can adhere to other bodies. Without attempting to 

 follow the author's minute anatomical description of this animal 

 we shall proceed to speak of their metamorphosis. By the sucker 

 already spoken of, they frequently attach themselves to the lymnea 

 or snail, around which they sport, and for a time as frequently de- 

 tach themselves again. When the period arrives for a farther 



