18 



OCCURRENCE 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 duodenaJis ; 

 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 Echinorhynchus 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. 1 In all 

 probability, this way of taking in nutriment by 

 endosmosis is not confined to the anenterous 



FIG. 11. A male Echi- , . , ,, ^ , 



norhynchus angustatus. ^Tms y but exists generally among Entozoa, 

 (The internal organs con- though it undergoes various modifications in 



sist of the sheath of the .,, , ,., 



proboscis, with retractor correspondence with the various differences of 

 muscle, lemniscus, and structure in the outer covering of the body. 



sexual organs. An in- .^ ., . . . Al -rT , 



testine is wanting.) From this point of view, the Entozoa may be 



1 In the Rhizocephalida (Sacculina, &c.) we have recently discovered a group 

 of ectoparasitic 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 respect to these interesting parasites see especially Kossman, 

 " Suctoria and Lepadidae : " Heidelberger Habilitationsschrift, 1873. 



