ORAL APPENDAGES ENTOZOA. 17 



contact with the air. The possession of these organs is an almost 

 exclusive attribute of ectoparasites ; for the Entozoa belong, with a few 

 exceptions, to the group of worms which breathe by means of the skin. 

 The Entozoa, besides having no special respiratory organs, are also with- 

 out pigment, the skin being whitish and transparent : in this they agree 

 with many other creatures, which, like themselves, are removed from the 

 influence of light. The ectoparasites, on the other hand, especially the 

 temporary parasites, agree in these respects with free-living animals. 



The modifications undergone by parasites, to adapt them to the 

 various conditions, are also to be noticed in the structure of the 

 mouth organs. 



Parasites upon the outer skin, of the higher Vertebrata at any rate, 

 can obtain no other nutriment than a more or less firm horny sub- 

 stance belonging partly to the epidermis and partly to the struc- 

 tures that originate from it ; it is needful, therefore, that they should 

 possess some apparatus strong enough to gnaw through these hard 

 tissues, and this we find, in the form of powerful jaws, in many lice, 

 and especially in the Mallophaga. In the same way, parasites that 

 feed upon the blood of their host must be able to bore through its epi- 

 dermis, in order to reach their food and then suck it up. In these 

 cases we find either mandibles, surrounded by a circular lip that plays 

 the part of a sucker, as in the leech (Fig. 9), or a boring apparatus, as 

 in the common lice, bugs, fleas, and mosquitoes, 

 which has the advantage of working rapidly, and 

 is therefore specially adapted to these parasites 

 which only visit their host for a short time. 



The necessity of a special mouth apparatus can 

 only be dispensed with in those ectoparasites that 

 live upon an animal which has a soft skin, as is 

 generally the case in aquatic animals. The para- 

 site, then, is provided with some contrivance that 

 enables it to suck; generally a pharynx, or some F i G .9.-^phalicend 

 muscular apparatus which allows of an alternate of Hirudo medidnaiis, 



. , . , , n i_i J.T , with the three mandibles 



widening and narrowing ot the mouth cavity, or at the base of the oral 

 which under other circumstances may cause merely CU P- 

 a peristaltic action. 



The Entozoa generally possess some apparatus of this kind in 

 contradistinction to the ectoparasites, and are but rarely provided 

 with jaws like the latter, except in a few cases, such as Uochmius duo- 

 denalis (Fig. 10), which, although parasitic in the intestine, lives upon 

 the blood of its host, and not upon the epithelial lining or contents 

 of the intestine ; and is in this respect, therefore, analogous to an 

 ectoparasite. Since most entoparasites are entirely nourished by 



