PLATODES. 269 



Class I.— PLATODES. 



Schneider, " Untersuchungen iiber Plathelminthen," Vierzehnier Bericht der oberhess 

 QesdUch. filr Natur- u. Heilkunde : Giesseii, 1873. 

 < Minot, "Studien an Turbellarien, Beitrage zur Kentniss der Plathelminthen," 

 Arieiten des zool.-zoot. Institutes in Wilrzhurg, Bd. iii., 1876-77. 



Animals with a more or less flat and short body, seldom ringed, 

 either with no appendages, or with organs of attachment in the form of 

 suckers and hooks, varying in number and arrangement. In a few 

 cases there are gills. Generally hermaphrodite, the young forms some- 

 times resemble the adults, bid sometimes differ from them, and exhibit an 

 alternation of generations before attaining sexual maturity. When 

 this takes place by budding, the sexual animals remain for a long time 

 united with their nurse in a polymorphic colony. 



The majority are temporary or stationary parasites, as the frequent 

 occurrence of fixing organs would lead one to suppose. 



The alimentary canal has a fleshy pharynx, discharging the 

 function of an organ of attachment or of capture, and showing, in 

 accordance witli the form of tlie body, a striking tendency to rami- 

 fication. In many cases the anus is absent, in others (endopara- 

 sites) even the whole canal, so that the ingestion of food takes 

 place by the surface of the body. Blood and blood-vessels are only 

 found in segmented fiat-worms. They have indeed been described also 

 in other forms, but this was due to a confusion with an often rami- 

 fying excretory apparatus which generally occurs in these organisms. 

 The nervous system consists of a ganglion lying in front of the mouth- 

 opening, or at the anterior end of the body, and of nerves running 

 from this, usually with two specially strong and well developed 

 lateral branches. In the segmented flat-worms these unite in the 

 middle line, and form a single chain of ganglia running below 

 the alimentary canal. The sense organs are usually but slightly 

 developed, and in the endoparasites are generally present only as 

 tactile organs. 



A body-cavity, usually present to contain the vegetative organs, 



is here wanting. These animals are parenchymatous worms (vers 



parenchymateux), i.e., organisms whose viscera are in continuity with 



the other tissues, and are directly embedded in the general connective, 



tissue substance of the body like the muscles and nerves. In other 



words, the splitting which usually separates the mesoderm into an 



outer somatic and an inner. splanchnic layer has not taken place, or if, 

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