DIFFERENT DEGREES OF PARASITISM. 93 



according to the degree to which their parasitism becomes stationary. 

 Likewise, also, the sensory perceptions, with their corresponding organs, 

 degenerate. The body loses its segmentation, and finally becomes 

 changed into a cylindrical mass, which not only swells considerably 

 under the pressure of the rapidly growing sexual organs, especially 

 the ovaria, but often becomes deformed in a most irregular manner. 

 Such extreme cases are exhibited among the Copepoda by the Ler- 

 naeadse, 1 among the Isopoda by the Entoniscidse, 2 which live an 

 entirely entozootic life. 



But even in these extreme cases the parasitic Crustacea possess, 

 in their young state, the same organization as do the allied free-living 

 forms, and, with a similar form, they lead also at first a similar life. 

 The transformation into the definitive condition is slow and gradual, 

 and is brought about by a metamorphosis which runs parallel with 

 every change in the relations of life. 3 That the metamorphosis is 

 retrogressive on the whole, and that it advances to different degrees 

 according to circumstances, has been mentioned above ; I will only 

 add that in correlation with a previously mentioned fact (p. 44) 

 it often reaches a higher degree in the female than in the male. 



In the same manner also, as in the case of the parasitic Crustacea, 

 the natural relations of the Gregarines, of the itch-mites, and of the 

 mosquitoes, may be determined to the free -living forms related 

 severally to each of them. But among the parasitic insects there are 

 forms in which the relations are less evident, and the intermediate 

 connecting links are wanting. For instance, the lice and fleas stand, 

 notwithstanding their large number of species, to a large extent 

 isolated from their related forms. They possess characteristics so 

 different that no connecting links have as yet been found, so that even 

 the systematic position of these animals appears in no way deter- 

 mined. The same is the case with the greater number of the so-called 

 intestinal worms. The groups Cestodes, Trematodes, and Acantho- 

 cephala consist entirely of parasites, although they differ from each 

 other in the degree of their parasitism, especially the Trematodes. 

 The tape- worms and Acanthocephala are capable only of a parasitic 

 life, through the want of a mouth and alimentary canal ; for a free 

 life presupposes the capacity of taking up nutritive substances into 

 the body directly by means of a permanent or temporary opening. 



Among the intestinal worms there is only a single group which 



1 C. Glaus, " Beobachtungen iiber Lernaeocera, &c. :" Marburg, 1866. 



2 Fr. Mliller, Archiv fur Naturyesch., Jahrg. xxviii., Bd. i., p. 10, 1862 ; Jenaische 

 Zeitschr., Bd. vi., p. 53, 1867 ; and Buchholz, Zeitschr. f. idss. Zool., Bd. xvi., p. 103, 

 1866. 



8 See Glaus, " Beitrage zur Kenntniss der Schmarotzerkrebse," Zeitschr. f. wiss. Zool., 

 Bd. xvi., p. 365, 1864. 



