84 THE DIFFERENT GRADES OF 



imal in which the worm lives as a parasite. Whether this is a 

 circulatory system, or one which serves to bring the surrounding 

 medium into more intimate contact with the internal tissues, in 

 order that it may be absorbed as a nutriment, is still a question 

 unsettled among naturalists. Whatever food is taken by the 

 Ttienia is introduced, no doubt, by some method analogous to 

 that by which a sponge absorbs water. When, therefore, the 

 joints are set free, there is merely a cutting in two of one of the 

 few organs which the body possesses ; and each of these individ- 

 ual segments consists, then, of a reproductive organ * and a part 

 of the system of the canals of the parent. The latter do not 

 serve any purpose whatever, since the joints soon cease to show 

 signs of life, and, in fact, decay not long after they are discharged 

 from the animal in which they lived, and have expelled the eggs 

 from the ovary. They are, functionally, egg-sacs. 



With this knowledge, then, of the structure of the joints of 

 Taenia, we are justified in looking upon them as enjoying the 

 remotest degree of individuality, when contrasted with that of 

 Myrianida; and were there not instances, among the different 

 kinds of intestinal worms, of all possible grades between these 

 two extremes, we should not hesitate to throw that of T^enia 

 altogether out of the category of individuality, and set it down 

 as one of the many known modes of laying eggs. 



Putting aside, now, these questionable forms of individuality, 

 which, by the way, I would say, I have discussed at pretty full 

 length for the sake of future reference, we have in such animals 

 as Hydra, (fig. 27,) Metridium, (fig. 28,) and Stentor, (fig. 30,) 

 examples of the formation of perfect individuals, without the 

 intervention of the egg-phase. I will state, moreover, that these 

 are by no means single instances, for there are several species 

 of Hydras, and the kinds of Anemones are numerous all over 

 the globe, whilst of the group of Protozoa, to which Stentor 

 belongs, there are hundreds of species in various lands, many 

 of which we know, from actual observation, do undergo this 



* Each joint is really compounded of twins, one a male and the other a 

 female. 



