P. 252, Insert heading “Family. Opisthorchiidæ, Braun, 1901,” above “Sub-family. Opisthorchiinæ, Looss, 1899.”
P. 351, description of fig. 255, line 3: for “Thoma” read “Thomas.”
P. 471, line 15 from bottom: for “alcohol 100 parts” read “alcohol 100 c.c.”
P. 472, line 11 from bottom: for “Or (2) 10 per cent. formalin,” read “Or (2) fix in hot 10 per cent. formalin.”
P. 493, line 21 from top: for “Conoy” read “Couvy.”
P. 589, line 2 from top: for “carnosa” read “carnaria.”
P. 620, line 15 from top: for “fo” read “of.”
P. 622, line 12 from bottom: delete comma after quantity.
P. 626, line 6 from bottom: delete comma after Mackie (1915).
P. 638: insert title “TREMATODES” above that of “Fascioliasis.”
P. 709, line 9 from bottom: omit second Pediculus capitis.
P. 748, line 8 from top: for “cytologica” read “cytological.”
P. 753, line 4 from bottom: for “Fercocercous” read “Furcocercous.”
P. 755 line 7: for “Oncocerca” read “Onchocerca.”
ON PARASITES IN GENERAL.
By the term PARASITES is understood living organisms which, for the purpose of procuring food, take up their abode, temporarily or permanently, on or within other living organisms. There are both plants and animals (Phytoparasites and Zoöparasites) which lead a parasitic life in or upon other plants and other animals.
Phytoparasites are not included in the following descriptions of the forms of parasitism, but a very large number of animal parasites (zoöparasites) are described. The number of the latter, as a rule, is very much underrated. How great a number of animal parasites exists may be gathered from the fact that all classes of animals are subject to them. Some of the larger groups, such as Sporozoa, Cestoda, Trematoda and Acanthocephala, consist entirely of parasitic species, and parasitism even occurs among the vertebrates (Myxine). It therefore follows that the characteristics of parasites lie, not in their structure, but in the manner of their existence.
Parasitism itself occurs in various ways and degrees. According to R. Leuckart, we should distinguish between OCCASIONAL (temporary) and PERMANENT (stationary) PARASITISM. Occasional parasites, such as the flea (Pulex irritans), the bed-bug (Cimex lectularius), the leech (Hirudo medicinalis), and others, only seek their “host” to obtain nourishment and find shelter while thus occupied. Without being bound to the host, they usually abandon the latter soon after the attainment of their object (Cimex, Hirudo), or they may remain on the body of their host throughout their entire development from the hatching of the egg (Pediculus). It follows from this mode of living that the occasional parasites become sometimes distinguishable from their free-living relatives, though only to a slight extent. It is, therefore, seldom difficult to determine the systematic position of temporary parasites from their structure.
In consequence of their mode of life, all these temporary parasites live on the external surface of the body of their host, though more rarely they take up their abode in cavities easily accessible from the exterior, such as the mouth, nose and gills. They are therefore frequently called Epizoa or Ectoparasites; but these designations do not cover only the temporary parasites, because numerous epizoa (as for instance the louse) are parasitic during their entire life.
In contradistinction to these temporary parasites, the permanent parasites obtain shelter as well as food from their host for a long period, sometimes during the entire course of their life. They do not seek their host only when requiring nourishment, but always remain with it, thus acquiring substantial protection. The permanent parasites, as a rule, live within the internal organs, preferably in those which are easily accessible from the exterior, such as the intestine, with its appendages. Nevertheless, permanent parasites are also found in separate organs and systems, such as the muscular and vascular systems, hollow bones and brain, while some live on the outer skin. Here again, the terms Entozoa and Endoparasites do not include all stationary parasites; to the latter, for instance, the lice belong, which pass all their life on the surface of the body of their host, where they find shelter and food and go through their entire development. The ectoparasitic trematodes, numerous insects, crustacea, and other animals live in the same manner.
All “Helminthes,” however, belong to the group of permanent parasites. This term is now applied to designate certain lowly worms which lead a parasitic life (intestinal worms); but they are not all so termed. For instance, the few parasitic Turbellaria are never classed with the helminthes, although closely related to them. The turbellarians, in fact, belong to a group of animals of which only a few members are parasitic, whereas the helminthes comprise those groups of worms of which all species (Cestoda, Trematoda, Acanthocephala), or at least the majority of species (Nematoda), are parasitic. Formerly the Linguatulidæ (Pentastoma) were classed with the helminthes because their existence is also endoparasitic, and because the shape of their body exhibits a great similarity to that of the true helminthes. Since the study of the development of the Linguatulidæ (P. J. van Beneden, 1848, and R. Leuckart, 1858) has demonstrated that they are really degenerate arachnoids, they have been separated from the helminthes.
It is hardly necessary to emphasize the fact that the helminthes or intestinal worms do not represent a systematic group of animals, but only a biological one, and that the helminthes can only be discussed in the same sense as land and water animals are mentioned, i.e., without conveying the idea of a classification in such a grouping. It is true that formerly this was universally done, but very soon the error of such a classification was recognized. Still, until the middle of last century, the helminthes were regarded as a systematic group, although C. E. v. Baer (1827) and F. S. Leuckart (1827) strenuously opposed this view. Under the active leadership of J. A. E. Goeze, J. G. H. Zeder, J. G. Bremser, K. A. Rudolphi and F. Dujardin, the knowledge of the helminthes (helminthology) developed into a special study, but unfortunately it lost all connection with zoology. It required the intervention of Carl Vogt to disestablish the helminthes as one class of animals, by uniting the various groups with those of the free-living animals most closely related to them (Platyhelminthes, Nemathelminthes).
Permanent parasitism in the course of time has caused animals adopting this mode of life to undergo considerable, sometimes even striking, bodily changes, permanent ectoparasites having as yet undergone least alteration. The latter sometimes bear so unmistakably the likeness to the group to which they belong, that even a superficial knowledge of their structure and appearance often suffices for the recognition of their systematic position. For instance, though the louse, like many decidedly temporary parasites, has lost its wings—a characteristic of insects—in consequence of parasitism, yet nobody would deny its insect nature; such also occurs in other temporary parasites (Cimex, Pulex). On the other hand, the changes in a number of permanent ectoparasites (such as parasitic Crustacea) are far more considerable, and correspond with those that have occurred in permanent endoparasites.
These alterations depend partly on retrogression and partly on the acquisition of new peculiarities. In the former case, the change consists in the loss of those organs which have become useless in a permanent parasitic condition of existence, such as wings in the louse, and the articulated extremities seen in the larval stage of parasitic Crustacea. The loss of these organs goes hand in hand with the cohesion of segments of