in INFLUENCE OF PARASITISM ON STRUCTURE 173 



entirely of parasitic forms, while most of the Turbellaria live freely. The transition 

 from the free life to the parasitic brings with it such far-reaching changes in the 

 conditions of existence that the original organisation, development, and life-history 

 of the animals must necessarily be strongly influenced by it. This influence can be 

 stated in a way which suits all cases in the animal kingdom where, in a naturally 

 demarcated animal group, parasitic forms appear side by side with free-living forms. 

 Similar variations in the conditions of existence have as a consequence similar varia- 

 tions in structure and development. 



We can, apart from fine shades of difference in manner of life, distinguish two 

 principal groups of parasites : (1) the Ectoparasites, which are parasitic on the outer 

 surface of other animals, and (2) Endoparasites, which are parasitic in the intestine 

 or other inner organs. The ectoparasites in many ways form the transition from 

 non-parasitic to endoparasitic animals, for they still retain relations to the outer 

 world which the latter have entirely given up. 



Parasitic life is the most convenient manner of life for the attainment of food. 

 Parasites feed at the expense of the juices or tissues of their hosts, which are 

 abundantly within their reach. Once on or in the host's body, it is of the greatest 

 utility for them to retain the position they have gained. Hence the numerous and 

 varied adaptations for the attachment of the body. In the Trematoda and Cestoda 

 we find suckers, hooks, and protrusible proboscides armed with barbed hooks and 

 other organs of adhesion. Many parasites possess a sucking apparatus to suck the 

 juices of the host. Trematoda suck with the oral sucker and pharynx the mucus on 

 the surface of the body, or the food pulp in the intestine, etc. 



The ectoparasitic Trematoda possess a well-developed alimentary canal, which is 

 often even richly branched ; in the endoparasitic forms, which are supplied with food 

 already partly dissolved, the work of digestion is facilitated. The intestine in endo- 

 parasitic Trematoda is reduced to two main branches or to a simple cfecum ; in the 

 Sporocyst generation, which multiplies parthenogenetically, it has become quite rudi- 

 mentary. Here feeding takes place simply by the diffusion of the juices of the host 

 through the outer skin of the parasite. The same is the case in the Cestoda, which 

 have entirely lost their alimentary canal. We may therefore state that progressive 

 accentuation of parasitism is accompanied by progressive reduction of the gastro-canal 

 system, ending in its entire disappearance. 



The capacity of active locomotion is generally of very little use to endoparasites. 

 We accordingly find in them that those parts which serve for locomotion, locomotive 

 organs and musculature, are more or less reduced. On the other hand many 

 ectoparasites (not indeed exactly Platodes) possess a well-developed capacity of 

 locomotion, which is of great importance to them, chiefly for the object of infecting 

 new hosts (e.g. the flea). Very many ectoparasites can, in fact, live a free life for 

 a time. The locomotory system and its musculature are therefore generally less 

 degenerated in them than in endoparasites. 



In consequence of the very limited locomotion of endoparasites the power of 

 directing themselves by special sensory organs is unnecessary, at least while para- 

 sitism lasts. The ectoparasitic Trematoda already are far more sparingly supplied 

 with sensory organs than the free-living Platodes. They still possess eyes, although 

 of a very simple sort. The endoparasitic Trematoda have lost even these sensory 

 organs, which occur only temporarily in the freely moving young stages of the 

 ciliated larvre and the CercaricK. In the Cestoda special sensory organs are altogether 

 wanting. 



The degree of development of the nervous system depends (1) on that of the 

 musculature, and (2) on that of the sensory organs. AVe thus understand the 

 gradual simplification of the nervous system, especially the sensory portion, from 

 the ectoparasitic Trematoda to the endoparasitic, and finally to the Cestoda. On 



