404 PHYSIOLOGICAL DEVELOPMENT. 



inhabiting certain Medusce, or those which nestle in the 

 branchial sac of the Lophius. After these may be named the 

 less injurious forms of parasites proper — those which, dis- 

 tinguished as Epizoa, fix themselves on the skins of their 

 hosts, permanently or temporarily, such as, of the one kind, 

 the Lerncea on fishes, and of the other kind the Tick on 

 mammals and birds. Then there come the other class of 

 parasites, most of them highly injurious, distinguished as 

 Entozoa, living within the bodies of their hosts, now in parts 

 of their alimentary canals, now on other of their mucous 

 surfaces, and now in various of their organs : these last two 

 groups being so numerous in their kinds that there are 

 commonly more species than one proper to each larger animal. 

 One stage further in the complication meets us in the para- 

 sites upon parasites. 



But now the general fact, to which these brief indications 

 are introductory, is that the use made of one organism by 

 another has been ever widening and becoming more involved. 

 Among plants utilization of the larger by the smaller — of 

 trees by epiphytes and parasites — must have arisen since the 

 times when the larger came into existence — times relatively 

 late in the course of organic evolution. Moreover most of 

 the plants which utilize others, either by climbing up them or 

 settling themselves high up on their stems or sucking their 

 juices, are phsenogams, and the plants they utilize are also ] 

 phasnogams; so that these innumerable interdependences 

 must have been established since the phsenogamic type has 

 become so predominant in respect of both size and kind. 

 Similarly among animals. Though there are many parasites 

 belonging, like the Trematodes, to very low classes, there are 

 many which belong to the Arthropoda, and, being degraded 

 forms of that class, must have come into existence after 

 Arthropods of considerable structure had been evolved. 

 Again, a large part of the animals infested by Epizoa and 

 Entozoa are vertebrates — many of the highest types ; and as 

 these are relatively modern all this parasitism must be of 



