16 ON PARASITES. 



The codfisli lays its 9,000,000 eggs, the shark its dozens. Those 

 of the codfish are naked and defenceless, whilst the others are care- 

 fully protected by a horny and persistent covering. The cercaria 

 marginata leaves its sporocyst and the body of the paludina where it 

 was generated, and swims in the surrounding water. Although 

 numerous animals have been examined, the particular species in which 

 it can develope into a distoma has not yet been discovered. Hence 

 the chances for the destruction of this parasite vastly exceed those 

 favorable to its final development. Accordingly this species of 

 cercaria is produced in great abundance, so great indeed, that they 

 often completely fill the testes and ovaries of the paludina in which 

 they are developed. 



Pathological conditions as well as those that are physiological 

 afford a nidus for the development of parasites. 



The class of parasites infesting the animal creation, to which by 

 far the greatest interest is attached, are the Entozoa. They have 

 received a corresponding degree of attention. The perfect adapta- 

 tion of all living beings to the circumstances in which they are in- 

 tended to pass their existence, is a never ending source of admira- 

 tion to the scientific observer. 



Turning to these Entozoa, we see purpose or function just as 

 strongly manifested as elsewhere. High intelligence has been pro- 

 vided for in the conditions of the air and dry land. Here, in the in- 

 terior of organisms, enshrouded in darkness, and in relation with 

 vitalized structures, what sort of organization would be expected ? 

 The intelligence required for the obtaining of their food is a minimum, 

 for their food is prepared at hand ; muscular activity of any kind is 

 as unnecessary as intelligence, with the absence of muscular develop- 

 ment a nervous apparatus is unnecessary. Pood is prepared already 

 elaborated, hence no digestive apparatus, or only a simple one is 

 required. 



Accordingly upon examination of these creatures we find no defi- 

 nite nervous system, no muscular development or a feeble one, no 

 brain, no digestive canal, (with exceptions.) Eyes these animals 

 have not, for they could see nothing if they possessed them. Ears 

 they have not, for no waves of sound ever approach them, a general 

 sense of touch it is to be presumed they have. They must be amen- 

 able to the great law of omne vivum ex ovo; i. e. they have a very 

 complete generative apparatus, which is always present in the perfect 



