[8] However, in the Protozoa there are examples of hereditary transmission of parasites, _e.g._, in the case of _Babesia_ (_Piroplasma_) _bovis_ and _Babesia canis_ in their invertebrate hosts (ticks); in _Crithidia melophagia_ and _Crithidia hyalommæ_; and in the case of _Spirochæta duttoni_ in its invertebrate host (a tick). _Derivation of Parasites._--Doubt now no longer exists as to the derivation of the temporary and of many of the stationary ectoparasites from free-living forms. This conclusion is founded on the circumstance that not only are there numerous intermediate degrees in the manner of living and feeding between predacious and parasitic animals, but that there is more or less uniformity in their structure. The differences that exist are easily explained as consequences of altered conditions of life. The case is more difficult in regard to groups that are exclusively parasitic (_Cestoda_, _Trematoda_, _Acanthocephala_, _Linguatulidæ_, and _Sporozoa_), or groups that are chiefly parasitic (_Nematoda_), because in these cases the gulf that divides these forms from free-living animals is wider. It is true that we know that the nearest relatives of the _Linguatulidæ_ are found amongst the _Arachnoidea_, and indeed in the _Acarina_; that, moreover, the structure and development of the _Sporozoa_ refers them to the _Protozoa_, and allows some of them to be regarded as the descendants of the lowest _Rhizopoda_. We know that the _Trematoda_, and through these the _Cestoda_, are closely related to the _Turbellaria_, from which they may be traced. The _Nematoda_, and still more the _Acanthocephala_, stand apart. This is less evident, however, in the Nematoda, for there are numerous free-living members of these from which it is possible that the parasitic species may be descended. Indeed, this seems more than probable if such examples as _Leptodera_, _Rhabdonema_ and _Strongyloides_ are taken into consideration, as well as the conditions of life of free-living nematodes. These mostly, if not exclusively, spend their lives in places where decomposing organic substances are present in quantities; some species attain maturity only in such localities, and there propagate very rapidly. Should the favourable conditions for feeding be changed, the animals seek out other localities, or they remain in the larval form for some time until more favourable conditions set in. It is comprehensible that such forms are very likely to adopt a parasitic manner of life which at first is facultative (_Leptodera_, _Anguillula_), but may be regarded as the transition to true parasitism. The great advantages attached to a parasitic life consist not only in protection, but also in the supply of suitable food, and consequently in the easier and greater production of eggs, and thus fully account for the gradual passage of facultative parasitism into true parasitism. In many forms the young stages live free for some time (_Strongylidæ_), in others, as is the case in