174 COMPARATIVE ANATOMY chap. 



account, however, of the strong development of the musculature of the organs of 

 adhesion the nervous system in relation with them is more or less strongly developed. 

 Compare the strong development of the nervous system in the head of the Cestoda 

 with its great reduction in the segments. 



Parasites seem to have a very slightly developed need for respiration. Judging 

 from what we find in other divisions of the animal kingdom, the respiratory organs 

 very often become degenerated, especially in endoparasites. The parasitic Platodes 

 have no covering of cilia. 



The excretory system in the Platode parasites is developed at least as strongly 

 as in the free-living forms. 



The genital organs also are as strongly developed, indeed even more strongly 

 developed, in the former than in the latter. Ripe Distoma or ripe segments of Cestoda 

 consist almost exclusively of the genital apparatus and the genital products. But to 

 this we shall return. 



We therefore see that with increasing accentuation of the parasitic mode of life 

 there is a proportional reduction of the sensory organs of the nervous systems, of the 

 special digestive system, of the locomotory organs, and also of the respiratory organs, 

 and thus a degeneration of all the organs except the genital and the excretory organs 

 and the organs for adhesion and sucking. 



The influence of the parasitic mode of life on the development, and generally on 

 the whole life-history of the parasite, is not less striking. 



If the parasite were to remain during life and in all stages of development 

 parasitic on or in the same host, it would perish when the latter died, and the 

 whole race to which it belongs would soon also perish. There must therefore be 

 some provision or other for the infection of new hosts. This infection takes place in 

 the simplest way in most ectoparasites. Many of these retain during youth their 

 free mode of life, so that they can themselves seek out their hosts. Others retain 

 throughout their power of free locomotion, and vividly recall in their mode of life the 

 beasts of prey. 



In the ectoparasitio Trematoda very little is known about the manner of infection 

 of new hosts, but we do know — and this is very important — that the course of 

 their development is direct and without intermediate hosts belonging to animal 

 groups different from that of the final host. In the endoparasites the life-histoiy 

 is, as we have seen, more complicated. But here also originally free-living young 

 forms must have provided for the spread of the individuals and the infection 

 of new hosts, and thus for the preservation of the race. The observation of free 

 scolices gives countenance to the presumption that originally a free-living young 

 form, a scolex, developed from the fertilised egg, and in some way or other again 

 found its way into the body of the final host. Most parasites are specialists, i.e. they 

 thrive only in the bodies of one or of a few definite animal species. It is certain, 

 however, that of their eggs or young forms only very few on the average reach the 

 bodies of true hosts ; many perish without reaching any host, many find their way to 

 the wrong place, go astray in the body of a host other than the usual one and there 

 perish, or they may for a time hold their ground and also, as experience shows, 

 develop somewhat further, never, or very seldom at any rate, attaining full develop- 

 ment. This perhaps throws light on the origin of development by means of inter- 

 mediate hosts. Carnivorous animals devour certain animals as their favourite 

 food ; the latter are themselves carnivorous or herbivorous. If the eggs or young of 

 a parasite accidentally reach the body of an animal which is the favourite 

 food of its proper host, and if they could there remain alive for a longer or shorter 

 time, the probability of their reaching in their new (intermediate) host the intestine 

 of their proper host would be greatly increased. This or some similar advantageous 

 manner of being smuggled into the body of the proper host might become established 



