INTRODUCTION O 



StrongylidfS, Trichinidce , Oxyurida, Trichocephalida, Filaridte, 

 Gordiida, Anguillulidte. 



IV. THORNHEADED-WORMS. ACANTHOCEPHALA. This group 

 embraces a small series of parasites, which, in general appear- 

 ance, resemble the nematode worms. They differ, however, 

 essentially, being, as the term indicates, furnished with spine- 

 covered heads. They are, moreover, destitute of digestive 

 organs. The species are entozoal in habit, abounding particu- 

 larly in fishes and reptiles. At present, all the known forms 

 are included in one family (Echinorhynchida), which also 

 comprises only a single genus. 



V. ANNELID PARASITES. SUCTORIA. In this category one 

 must place all such suctorial annelids as affix themselves 

 to hosts for a longer 'or shorter period. Many of the leech- 

 like parasites (Clepsinidte, and especially Mallacobdellidce) 

 remind one of certain flukes (Tristoma, &c.) possessing ectozoal 

 habits ; whilst the leeches, properly so called, afford instances 

 of the passage from a semi-parasitic to what has been called the 

 free parasitic mode of existence. In tropical countries these 

 creatures very readily attach themselves to man and animals, 

 often creating severe distress. The genera Glepsine and 

 Hamocharis attack mollusks and fishes respectively. The 

 species are all ectoparasitic and exceedingly numerous. They 

 cannot be described in this work. 



VI. ARACHNID PARASITES. ARACHNIDA (part of). The great 

 class of articulated, limb-jointed, or, more strictly, arthro- 

 podous animals, includes a variety of parasites. The mites, 

 true ticks, and such like creatures, belong to this group. 

 Some few of them are entozoal in habit, others are only par- 

 tially so, whilst the majority are entirely ectozoal. Of the 

 two great sections of Arachnida, namely, Pulmonaria and Tra- 

 chearia, the latter alone contains strictly parasitic forms. The 

 parasitic species belong to the following families : Pentasto- 

 mida, Pycnogonida, Ixodida, Acaridte, Gamasida, Hydrach- 

 nidce, Solpugida. The parasitism of some of the species is 

 very partial or slight. Thus, certain of the water mites, in their 

 juvenile state, dwell on aquatic insects only ; and the tick-like 

 Gamasidas occur upon dung-beetles. The other ectozoal species 

 attack vertebrated animals, and several attach themselves to 

 man himself. The whale lice (Gy amides) are here included in 

 the Pycnogonida, though often placed by zoologists with the 

 Crustaceans. 



