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Class V. — ACANTHOCEPHALI. 



We again enter the dominion of parasitism. The Acanthocephali, or barbed- 

 headed worms, comprise the single genus Echinorhynchus, a. genus ricli in parasites 

 whicli infest the liigher animals. Of them all the best known is 

 the Echinorhynchns yiyas, which is especially inimical to the pig, 

 but does not hesitate to make its home in the intestines of many 

 iither species of mammals, and even, it is said, in man. It grows 

 lo over a foot in length, and is, towards the middle of its body at 

 least, thicker than a lead pencil. Other species abound in fishes, 

 big and little. All Echinorhynchi (there are over one hundred spe- 

 cies known) have an elongated sack-like body, without any diges- 

 tive canal. Their essential external characteristic is the short, 

 more or less nearly cylindrical proboscis, which is armed with 

 several rows of hooks or barbs, whicii, like those of the tajie- 

 worms, serve to anchor the parasite in the tissues of its host. The 

 eggs are generally spindle-shaped, and are retained in the body of 

 tlie mother until the embryo is develo])ed ; the embryo is enclose<l 

 ill several membranes, and has a bilateral armature of thorns around 

 tlio anterior extreiliity. The embryos are su[)posed to pass the 

 larval state in some aquatic host; but the life history of these ani- 

 mals is still very imperfectly elucidated, therefore it does not 

 apjiear desirable to attempt a fuller account of the migrations and 

 metamorphoses of these parasites, because such an endeavor wcjuld 

 lead only to an unsatisfactory compilation of fragments. 



The internal anatomy of the Acanthocephali has been quite 

 thoroughly studied, but we have thereby only gained a knowledge 

 of the extraordinary singularities of their organization, without ob- 

 taining any real clew as to their afMnities with other worms. The 

 most we can safely assert is that they present .some resemblances 

 with the nematods, after which we have accordingly placed them 

 in tiie system here ado])ted. 



Class VI. — CH^TOGNATHI. 



Along the Atlantic seaboard the skimming net may gather from the oceaii'.s sur- 

 face considerable numbers of little elongated animals, perhaps half an inch or more in 

 leiigtli, .and somewhat javelin-like in shape. They move with quick jerks, which are 

 highly characteristic. Ex.amined more carefully they are seen to be almost pearly 

 wiiite in color, and quite translucent. Their form is nearly rod-like, with lateral fins; 

 the anterior end is enlarged to make tiie iie.ad, which is further characterized by two 

 dark s]iecks, the ])igmcnted eye.s, and two groups of curving hooks, acting as jaws. On 

 each side of the body is a horizontal membrane or ))rojecting fin ; this lateral fin varies 

 considerably in size; and outline in the different species, liut is always most develojied 

 on the posterior half of the body, but is often subdivided, and in that case forms an 

 anterior and posterior fin. A similar fin projects around ihv. tail, and is prob.ably 

 merely a specialized ])art of tlie horizontal fin, which we ma}' assumes to have extended 



