On Flulces. 27 



wider view in the matter, it is obvious that the distribution of 

 internal parasites, throughout space, must be co-equal and co- 

 extensive with the geographical range of the animals in which 

 they dwell ; and it also follows that they will have acquired a 

 corresponding distribution in time. Their bathymetric position 

 or distribution in height and depth in relation to our planet, will 

 also accord with that of the infested creatures ; in short, the 

 length, breadth, and area of their geological and geographical 

 range will be identical with that of the vertebrate groups 

 whose individual members they inhabit. 



This subject becomes yet more strikingly suggestive when 

 we take into consideration the complicated facts and phenomena 

 which the various phases of parasite development unfold ; for 

 during their larval wanderings in search of a final resting-place 

 which shall prove suitable to their adult condition, they pro- 

 visionally occupy the bodies of different kinds of evertebrata; 

 and in order to complete the genetic cycle of the parasite's life, 

 there must needs be, of course, a contemporary existence of 

 both vertebrate and evertebrate types, a concurrence which 

 surely no reasonable person would ascribe to fortuitous circum- 

 stances. Further into this speculative inquiry we do not now 

 enter, having purposely suspended our record of the origin, 

 growth, and migration of the young flukes, until we have dis- 

 cussed the features of their adult structure. Meanwhile, how- 

 ever, it is but fair to acknowledge that we have diligently 

 sought for a more practical evidence of the existence of internal 

 parasites in ancient times. This we have done by scraping 

 down portions of fossil excrement, and submitting them to 

 microscopic observation, in the hope of possibly stumbling 

 upon a parasite's hook or spine. Success in this experiment 

 would have enabled us triumphantly to vindicate the force of 

 our persuasion as to their pre-adamite creation; but as the 

 question now stands, few naturalists can doubt their former 

 prevalence. Of course, a searching like the above, can hardly 

 ever prove effective, for the delicacy of their tissues, the mi- 

 nuteness of their bulk, and more particularly, the extreme rarity 

 of their being mixed up with the eliminated products of the 

 alimentary canal, are considerations which almost warn us of 

 the hopelessness of such investigation. If, however, real colo- 

 lites, or fossil sections of the digestive tube of any of the larger 

 extinct vertebrates could be obtained, then we should not en- 

 tirely despair of recording ocular proof of the occurrence of 

 Entozoa in the secondary and tertiary epochs. 



In regard to the number of existing species of Trematoda, 

 no very accurate estimate can be formed. The writer of this 

 article, not very long ago, made a special investigation, partly 

 with the view of determining this point, and the results of this 



