18 



OCCUERENCE OF PARASITES. 



the fluid or semi-fluid substances which surround them, the presence 

 of the above-mentioned sucking organs is quite intelligible ; they are 



Fig. 10. — Cephalic extremity of Dochmius duodendlis ; 

 profile and front view. 



not, however, absolutely necessary. Many 

 Entozoa have no muscular pharynx, and are some- 

 times even entirely destitute of an alimentary 

 canal, and must absorb their food through the 

 surface of the body, after the fashion of a plant, 

 without the action of any further process. 

 The Cestodes and UcJiinorhynchus belong to 

 this class, and their outer skin possesses the 

 requisite permeability to a high degree, as may be 

 easily proved by placing the animals in water, 

 when they swell up rapidly. Of course, it is 

 only substances dissolved in fluids that can 

 find their way into the interior of the bodies 

 of these parasites ; but they usually live in situ- 

 ations where they are surrounded by nutritive 

 fluids to such an extent that they may be re- 

 garded as almost swimming in them.^ In all 

 probability, this way of taking in nutriment by 

 endosmosis is not confined to the anenterous 

 forms, but exists generally among Entozoa, 

 though it undergoes various modifications in 

 correspondence with the various differences of 

 structure in the outer covering of the body. 

 Erom this point of view, the Entozoa may be 



^ In the Ehizooephalida {Sacculma, &o.) we have recently discovered a group 

 of eotoparasitic Crustacea that have no alimentary canal. They obtain their food 

 like plants, by a number of branched prolongations, which pass through the body of their 

 host and ramify in its intestine. They are found generally on the ventral surface of the 

 abdomen of crabs. With refp^fj^^ej^^'j^f^gflf^asites see especially Kossman, 

 " Suctoria and Lepadidse : " HeulelbergerTlabilitationsschrift, 1873. 



Pig. 11.— a male Echi- 

 norhynchus angustatus. 

 (The internal organs con- 

 sist of the sheath of the 

 proboscis, with retractor 

 muscle, lemniscus, and 

 sexual organs. An in- 

 testine is wanting.) 



